These motors are much different from the 45" engines in army bikes and servi-cars. Not many parts will interchange. The WR was built for low-drag (meaning low resistance) and less friction for increased power. If you can't get the power from cubic inches, look elsewhere.
This cover blocks off the original timer(or magneto) position. Some racers install a specially manufactured tach-drive here - for a gear driven, cable driven rev counter.
The Wico single fire magneto runs a rotor (just like your car) and fires each cylinder separately. It's mounted horizontally to reduce drag between gears. Whenever you have a bevel gear(vertical) driving a bevel gear(horizontal) you have increased friction, and drag (robbing power).
The MR-3 is a short-barrel Linkert racing version. It has no choke. The choke would block the way for air, restricting flow. It utilizes an adjustable high-speed main jet circuit with a needle. The slotted set screws in the carburetor body(inside the float bowl) are removed to facilitate higher fuel flow.
These carburetors run best at wide open throttle. It's common for the engine to run a bit rough, babble and snort . . . until it cleans out - then it's HOLD ON !
The bombsite is designed to assist in mixing and directing fuel and air vapor. Oxygen and flammable liquid to create the largest, controlled explosion possible. Cylinder fins are larger for improved and consistent cooling - for long races on hot summer nights !
The tiny engine sprocket keeps the rpms in the power band, gets your ass pullin' out of corners . . . and gets you up and over the steep hills. In 1952 when the H-D factory came out with the more powerful KR racer . . . the WRs still took fast times, and feature wins. It took a few years for the WR to lose it's competitive status. To hear a WR wound out tight is music to my ears. Nothing is sweeter. Classic side valve performance never goes out-of-style.