which MDC RS-3 do You have?
original had cast metal trucks,22-1 gear ratio,and Athearn axle gears.
also had an Athearn motor with flywheels.
2nd version had a complete Proto drive from the FA-2.
both versions can use Athearn SD40-2 axle gears.
the 3rd version is the Athearn modified version,so that should use the
SD40-2 gears also.
these critters have been around since the late 80s,so there should be plenty
of them out there for use as parts sources.
Spikre