Hey snellemin, one of those cheapo Chinese 4-wheelers from Traders Village would be an interesting setup. They would probably hold up to power a little better.
Only been there once for the VW "carshow" they had. Gonna hafta go again and look for those chinese 4 wheelers.
My son has 2 of the 12 volt John Deer Gator riding toys. They are made by Peg Perego. The stock motors are some kind of a 550 I believe. It goes really fast on 24 volts but the motors don't last long. I think a 775 brushed motor would do well in there. The motor mount holes would need to be enlarged to 1 1/8. I also eliminated the "shifter" that changes between reverse, 1st., and 2nd. gear.
Get some giant rubber bands to put around the tires! Needs a major dose of traction. Not sure if the rubberbands would work or not but that thing has some nasty push turning on the concrete and wouldn't hurt from some added traction to the back tires either! Now you just need to put a neu and a MMM in there and he could have some real fun!