Could be one of three things first your coil, second the crankcase seals are leaking when our crankcase seals went the engine would be awesome for about 3 laps and then drop off massively once warm, we replaced the seals and it was fine again, third item could be the piston ring has stuck all the way round and not just at the back where it needs to be stuck. The first one to check would be the coil borrow another coil of a friend or someone at the track and try it. To check the crank case seals you can do this with a carb tester and it is not difficult remove the flywheel and clutch assembly and carb, leave the spark plug in tight now block off the restrictor and inlet port with the red plastic bungs engine builders give you the engine back with. Make sure they are pushed in tight. Now put your carb tester on the pulse pipe fitting in the crank case and pump away once you have a reasonable amount of pressure on the gauge spray brake cleaner around the crankcase seals and look for tiny bubbles leaking from the seals if you see any bubbles at all replace the seals. You do not have to take the engine apart to replace the seals just carefully pick them out and push new seals over the crank into place. To check the piston ring just take the head and barrel off and the ring should be free 3/4 around the piston diameter and only stuck at the back off the piston overt the exhaust port. if it is stuck just carefully unstuck the ring at the front / sides and drop a bit of 3in1 oil on it and it will be fine
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