Bill, SR&RL 4 did get the Model A engine when the railbus was rebuilt at Edaville. You are right about Model A parts, they are easy to get with a number of companies, including Ford still supplying them.
As I mentioned in a previous post, SR&RL railcar 3, was built in 1934. It started with the frame of the old railbus 3 which was a Model T. The frame was shortened to be the same length as railcar #2 but the new #3 was built using Model A parts. The parts included the solid (slide out) windshield, arched visor, cowl, hood and radiator shell. The engine and transmission was changed to a Model A drive train but the car kept the original axles. The other equipment kept from the railbus was the Model T headlights. This was probably done because most Model T headlights are designed with a long mount/bolt to fasten the light onto the front plate so they sit down near the bottom of the radiator. The first SR&RL Model T, railcar 1 was an inside frame car that only lasted for 2 years. The rest of the fleet was built with an outside frame which made the cars track better. The outside frame extended past the radiator, giving a platform where the Model T lights were mounted. Model A headlights do not have the long bolt because they are mounted on a bar that goes between the fenders. There are no fenders on an outside frame car so there's no place to mount the headlight bar. The original frame would have had the headlights still intact, waiting to be hooked to the Model A wiring harness. If the SR&RL bought a complete Model A and parted it out to build the railcar, management probably sold everything that was left to recover some of the railroad's expense. The Model A headlights would have been sold since they were newer and more valuable. Selling all the un-need parts would have been common practice during the Great Depression.