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http://www.telegraph.co.uk/motoring/classiccars/9108430/The-ethanol-threat-to-classic-cars-and-bikes.html

where does he get fuel from?
you could try changing fuel pipes as big problem with ethanol eating rubber pipes.

right carb? jetted for new exhaust??

good luck too you, these are the worst problems!!
sounds like something heating up after time, could be piston growing and semi seizing (insufficient clearance) . on a scoot could be electric choke...