my buddy just got done rebuilding his 87 banshee,
he went 80 over plus added a cool head,got it all back together and it fired right up.he let it run for 10 minutes and then cool down,he did this twice as advised by the place who he took cylinders to,sounded good no problems.2 days later
we went out for a nice slow ride(1st since the rebuild)and all of the sudden maybe 1/2hr later it started losing power, so he killed it to check it out and now he cant even kick it over. the kick starter will not budge,almost like its jammed
anyone ever have this problem or have any ideas what might have caused it?

 

Pull the head and take a look.

 

This sounds like siezing (when the pisyon heats -and BTW expands- and stripes the cyl, and finally gets stuck if you keep running the engine).
That almost happened to me on a 125KX (MX Bike). Same thing. The bike was difficult to start, but ran fine, and suddenly it lost most of its power. I stoppped everything, and once home, I opened the head, and dicovered awful scratches on the cyl. , slightly bent head (due to the heat) and destroyed piston (the rings were floating in their devoted space). If I had continued, it would have siezed...
For the curious ones, all this happened because my water pump gasket was finished, and all the cooling water went in the engine (cranckcase)...

For you, maybe your pistons are too big for the sleeve you have (maybe?)

 

You can also take off the pipes and look inside the exhaust port and see whats going on. It will save some money on a head gasket as well. Plus you can see what the rings are doing.