Without spending more than FIFTY BUCKS! I have no idea. I would say a YZ250F but that will cost ya more than fifty bucks. You could try an older 125 or something like that. But if you don't want to spend more than fifty bucks you dont have a whole lot of options.
dang rich you must read EVERYTHING! I have seen your posts all over the place. I thought it was you that didn't like mini's but it must have been Eric???
Anyways if you don't know who Rich is do a search on his posts...They are most definetley worth a read. And he is spot on here too. You cannot do much with $50. I see all the time people wanting big bore kits for their kids Cobra but haven't replaced rings in eons. In fact Cobra went from a single ring piston to a double ring piston because people didn't change rings often enough and were complaining about all that maintenance they had to do.