Resin was one of my early thoughts. The original Nylon is pretty tough, and they kept breaking gears. (I think he was overvolting it) He tried replacement boxes but they just broke immediately, he managed to get a couple of original gears at $80 a piece, but they didn't last long either.

I think the right answer would have been to replace the motor with something that had a higher Kv and done a belt drive. (like electric skate parts with a little more ratio)