Back in the day, I remember having a KX/XR80 conversion with a Powroll 95cc kit.  It was so much fun, that little motor ripped.  It was my first mini bike ever.  Well, fast forward 30 plus years and Powroll is no longer around; however thanks to Vintageworks140 on Ebay, you can still get a XR80 big bore kit. 

You can buy them here:  Honda XR80 Big Bore Kit