I was looking at the 250cc two-stroke cart enginge that yamaha has out, and it looks to based off the yz250 powerplant. I doubt it is, but it gave me some ideas...

 

I'm certainly no expert on karts, but as I understand it, they are very well modded motocross motors. So no counterbalancer.

Also, most kart motors run in a very narrow RPM range, not a great choice for much heavier quads...