Ok, I know it's the wrong forum--but there really isn't a correct forum for the question. I did post the same thing in the sport bike forum btw. The Kawi forums don't have an obvious match. I've got a couple of old honda mx chassis laying around getting dusty ('85 CR500R and '00 CR125R), and thought it might be fun to put a liquid cooled side by side twin fourstroke in one or the other and show up to the track and ride it in the vet classes. From what I can find out online, either engine will pull the bike around pretty well, but the 500 engine is at least 80 lbs heavier than the 250 (which is certainly heavier than a single cylinder 2-stroke engine). I don't know the dimensions of either engine, but I would guess either would fit in the 500 chassis. Not so sure about the 125 chassis. Anyone seen anything like this? And what do you think? Would it be fun to ride--or just a big pain and waste of time? I've seen pictures of a guy riding an mx bike with an air cooled older inline 4 cylinder--actually jumping the thing. He must have gorilla springs in the forks... Obviously I'm bored at work...but I've actually been thinking about this conversion for awhile.