By the looks of it, the motor is 28mm x 47mm(ish)... (edit... after a closer look, it may be a 24mm diameter motor) that's the size of a feigao 380S motor. A medusa 28-40 of a neu 1107 would probably be ideal. But I'm with Linc on the diffs. Traxxas had better have done their homework on this or else they will be little more than pretty paper weights. I'll be sticking to my detta st until these trucks prove themselves reliable.
On another note, those ugly red plastic push rods definitely need to go...
on an even further note.... I wonder if traxxas will eventually come out with a mini summit crawler?