There's a stack of wiper and actuator motors here that I came across in my prelim search for track drive motors.
http://store.amequipment.com/.
They have a engineering drawing of each each, showing the dims and output etc etc. They also have LH and RH drives which overcomes the problem of reversing the polarity in one in same-drive side pair, given the (possible) difference in speed in reverse speed of one vs forward speed of the other. I:e, I saw on the net a guy who did that and one ran at 50rpm reversed while the other ran at 60rpm forward. Of course these could be bought anywhere, I just found engineering drawings and dims real handy in working out room in the frame.