It is a major project. You will need to shorten and narrow the swingarm, weld on shock tabs and relocate the upper shock mount. I looked into it then found this modified frame on ebay. It can be done, but seems like a lot of work. You might be better off getting a BBR swingarm and shock that comes with a relocating bracket that just bolts in, then add a KX65 or other aftermarket front end.
Here are some pics of the frame I bought on Ebay:
I works pretty well for adult (175lb) rider but a bit of a pogo stick for my 75lb kid.
Good luck