seems a strang fault you have paul .
i take there is no one near that you could swap parts with to try such as carb or coil
did you blow the carb out with compressed air ,what about water in fuel ? ,
if its a mis fire would think its electrical rather than fuel which would be more like loss of power and poor running
what about running it with the tank off to see if its tracking out ,or spraying everything with wd40
try re routing coil lead if poss . removing kill switch?
i take it you are no where near the northeast or you could try someparts of our bike
hope you get to the bottom of this , sorry if i am telling things to do that you have tried already
good luck mark