Since we all agree that the China pit bike engines are copied from the Honda 50-70 case, We can talk apples to apples. I'm saying that I think the 90 and the 110 China pit bike engine share a crank but have different bores. If that's true (and I'm not sure), then this kit would be a bolt-on deal:
http://www.ebay.com/itm/52mm-Cylinde...lYqmHJ&vxp=mtr
The price is certainly right.