A stock model 75 Stearman is pretty much a stock model 75 Stearman, and the variations are few, and relatively minor. The exceptions would be the relatively rare Jacobs powered and Lycoming 300 HP versions.
Even though the 450 Stearman is defined in the Type Certificate Data Sheet (TCDS) no two 450's are exactly alike. The differences are numerous, and significant.
I have outlined below most of the more common variations you will encounter on any particular 450. These variations most often reflect the different purposes to which the airplane has been put. From the duster & sprayer configuration, to the airshow mount, each 450 has a unique personality.

Certification

450 Stearmans can be found in any of three airworthiness categories: Restricted, Experimental, and Standard. Before purchasing or building, or even riding in a modified airplane, you should know the appropriate FAA regulations governing each airworthiness category.
Stearmans survive today because most all of them were converted to a duster or sprayer configuration, and assigned to the Restricted Airworthiness certificate. This certification severly limits the utility of the aircraft. Likewise the Experimental category, which is generally assigned to an aircraft that has been highly modified, usually for airshow or aerobatic performances.
The gold standard for a 450 Stearman is to be assigned the Standard Airworthiness Certificate. Such an aircraft has the least amount of limitations, and the greatest flexibility, and therefore value.

Engines

The TCDS (A-743) indicates that the acceptable powerplant for the 450 Stearman (IB75A) is the Pratt & Whitney R-985 AN-1 or AN-3 engine. Specifically excluded is the popular variation of the R-985 AN14-B engine. This engine was developed for the Beech model 18 using the Hamilton Standard fully feathering propellers, and has among other minor changes, a hollow crankshaft to accomodate the feathering process.
A number of 450 Stearman have been fitted with the AN14-B engine which has been modifed back to the AN-3 standard internally. If you get involved with such an airplane, you may want to check the logbook entries thoroughly.

Propellers

Again, per the TCDS, two propellers are approved for the 450. The Hamilton Standard ground adjustable 5406-AL hub with IC1-12 through IC1-14 blades, and the Hamilton Standard controllable 2D30 hub with 6101-12 through -20 blades (the dash number indicating the lenght of the blade, the -12 being the longest).
The 2D30 choice is favored, and is actually also found on higher horsepower Pratt & Whitney applications, such as on the North American AT-6. You can recognize the 2D30 unit by the counterweights projecting from the hub.

This is the 2D30 hub

The combination of the R-985 and the Ham Standard 2D30 is one of, if not the most robust and bullet-proof combinations ever produced in aviation. This is the very heart of the 450 Stearman.

Engine mounts

In the mid 1940's when the 450 Stearman was invented, the most common process was to take a firewall-forward assembly from a surplus Vultee BT-13, modify the engine mount a bit, and bolt it on the Stearm frame. BT-13's were available in great quantity, and could be had for very little as there was no real use for the airplane in the civilian market.
The only other airplane that was built in quantity that used the R-985 engine was the model 18 Twin Beech.
Because of the foregoing, two fundamental variations of engine mounts are used to mount the R-985 to the Stearman fuselage. The first is a modifed BT-13 type mount, and the second is a modified Beech 18 type mount.
The two types of mounts are easily identified. The Beech mounting ring bolts directly to the engine with no rubber cushioning at all. There is a simple hole in the mounting ring through which a bolt secures the engine.

Beech 18 type mount on the left, and Serv Aero BT-13 type short mount. The closeup shows the mounting method at the firewall. Last photo is the long mount.
A third type of mount, is a combination of Beech 18 and BT-13 mounts. It has the 4 Lord type mounts at the firewall, and the mounting ring incorporates rubber isolation blocks at the engine.

The BT-13 mounting ring incorporates 9 rubber isolation bushing assemblies. On this mount the engine is effectively isolated from the mount at the ring, and four metal to metal bolt attach points are secured to the original Stearman mount points. Generally, the frame attach studs are modified from the stock 7/16" up to a 1/2" stud for the heavier engine.
The Beech mount isolates the engine and its vibration from the airframe at 4 Lord-type mount bushings near the fuselage frame. The Beech 18 mount is modified from the original configuration with 3 lord type mounts at the firewall, 1 each on the upper left and right corner, and one mount at the 6 o'clock position, to 4 mounts at the stock Stearman location.
All these mounting systems have an associated STC. Of all the mounting systems, the most popular are those created by Serv Aero out of Salinas, California. They still produce and will install the STC'd 450 conversion.
Over the years, there have been some variations on the BT-13 type mount constructed by Serv Aero and other STC holders. The basic variation is in the length of the 4 mounting legs. The BT-13 ring remains unchanged on all.
On the shorter mount, when installed, the inspection covers for the magnetos are just about 1.5" from the firewall. On the versions with the longer legs, often called the long mounts, there is more distance between the firewall and the back of the engine accessories.

Cowlings
Few if any of the original duster conversions used an engine cowling. When the dusters and sprayers were converted, cowls were used to make the installation of the R-985 aerodynamically cleaner, and help promote better cooling. In addition to the outer cowling, each type of cowl requires supporting installations of intake systems, compatible exhaust, and metal shielding between the engine and it's accessories, commonly called a dishpan.
Availability of the BT-13 assemblies made them a prime candidate for the Stearman, as do the Beech 18 cowls. The BT-13 assembly requires little modification for installation, where the Beech 18 system requires some type of modification to accomodate for the lower cowl flap area as it mates to the flat configuration of the Stearman firewall.
Variations on the Beech 18 cowling include modifying the lower cowl half to exclude the cowl flaps, shortening the cowl and adding a static closure similar to the stock Stearman installation, and some retain the cowl flaps in both functional and non-functional configurations.
A third type of cowl is no cowl at all, but installation of a speed ring. These were developed in the 1920's and 1930's to aid in cooling air flow, and they also permit increased speed by cutting turbulence and drag around the engine. Speed rings are found on many Stearman, and are custom made for the installation.
It is worth mentioning that Jacobs and Lycoming powered Stearman may also have cowlings fitted, usually adapted from Cessna T50 installations.

BT-13 cowl, Beech 18 cowl with no cowl flaps, Beech cowl shortened, and a speed ring.

Exhaust
Three types of exhaust are typically fitted to the 450 Stearman. The BT-13, Beech 18, and the Broussard appear to be the most popular. All three of these require modifications to adapt to the fuselage and intake systems in use.

Typical BT-13 type exhaust, Broussard exhaust, and unique Beech Staggerwing exhaust.

Oil tank and cooler

Wings & Ailerons

Tailwheel

Wheels and brakes

The BT-13 single shoe brake, automotive master cylinders, the Redline disc brake.

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