The Banebot gearbox, as well the Portugese REC, the UK Saturn and Neptune (and grandfathers of'm, de GR01 and 02) are all based on something easy and simple to get, batterydrills. The only thing those do is replace the plastic houding with aluminium supports.

Also, pinions, to fit on 3.175mm shafts are easy to find. It's a standard in the RC car world.