I have been thinking over doing the same thing using drives from the newer battery drills with vari-speed controls.Some of the drills on the market place today are powerfull and cheap in price.The biggest problem is cutting them down to size and making a proper way to mount them in the hull.There are also very small and also cheap 90 degree drives that will seem to work for the drive shafts.
I think it may also be possible to use the exsisting triger speed control and reverse control with servo's,doing some moddifications to operate these with servo's.I have a non-RC Stuart on it's way to me and this is what I had planned on trying to use for it.
BIGMIG

P.S. Isn't figuring gear ratio just as simple as counting the amount of turns from the drive pinion gear it takes to turn the drive shaft one turn.(assuming you are trying to figure this on an existing gear box).?