Be on the lookout for older classic go karts, but not the high dollar ones--just the ones with minimal tubing on the frames. "Less is more" in this regard and the suspension arms front and rear, plus the "artful" tube designs like on your Yerf-Dog do add weight.
I just prefer the 6.5 212 engines because of the plentiful hop-ups available, but a 420 would help.. Sorry your kart is such a pig.