Watch the spacing, to line up the sprocket on the Lifan with the rear sprocket, some you have to shim. From what I have talked to people, you are just as good to buy the full 88kit as you are the 125 lifan. I have not tried this to prove it, but some Utah guys have. You do not have to stretch the frame for the 125 Lifan, if the motor hits the fender, your frame is bent and you need to see the thread on how to fix it. You will either have to cut the rear fenders or use the pull start conversion like rush is planning on. Also find a CRF50 exhaust, the stocker is just going to cork up the power your adding with the new motor. If you know an exhaust guy, supposedly you can make a slight bend and make the CRF exhaust work like Aaron B did.
You should look at the 150cc motors.
http://shop.pitsterpro.com/custombuilt150.aspx