Your best bet for replacement parts is to find a junker at a swap meet. I should know – i just sold two FM C Liners in working condition at a swap meet for a dollar each last year! I had a bag of five FM shells and I sold the whole bag for a dollar. I had a very popular table at that swap meet.
Also be aware that the AHM EMD BL2 used the same trucks (!) and motor as the FM so if you find one of those at a swap meet instead they should work fine.
I do believe the FM went through some design changes over the years but the one in the diagram above represents what I believe to be the most common version.
Part 13 is a pin that goes through the side walls of the chassis and the plate on the bottom of the motor. The wires are soldered to clips that go under the brush springs and are not permanently attached. Just taks a little finagling to get apart and back together.