sounds like you have plenty to go on with! just do one thing at a time and test, also start with the easy stuff ie head gasket throw another one in if it don't work no real time or money waste, on that note pretty much most low down pulling engines are a low comp motor, smaller carby is a good idea, and yes the exhaust is a important tunning device, we had a junior in sidecars who could not come to terms with the pokey little 2 stoke so we fitted a reducer washer on the end of the muffler as he got used to it we drilled out the washer, at the end removed the reducer, funny thing is 12 months later they win the state titles/champion ship on the same bike
