putting a 250 into a 125 wouldn't be much different, I think the 125 has a couple of inch shorter swingarm. There was a 500 stuffed into a KX 80 floating around on ebay for awhile. Not sure but I would think that the 80/250 would be pretty worthless other than a novelty, it would have to be hard to ride being so short and the small tires would probably deflect off everything. If you want something different maybe do a shifter kart with the 250 motor.
