A decent motor for a smaller bot are the Fisher Price motors which you can get at a PowerWheels service Center I think they cost us about $16 a piece. You could also buy some checp motors for RC type equipment at
www.towerhobbies.com in the 540 size. The fisher price have a standard pinion which you can mate to fairly easily, most of the motors from tower you supply your own pinion normally with a set screw. The BOSCH motors are very powerful and you can buy them at a BOSCH repair center with or without the planetary gearbox. If you need to know more either post or send one of us an email someone should get back within a day or so.
matthew.reiland@gm.com