buy a new top end kit, (piston,bearing,gaskets,wrist pin and clips) clean your powervalve, you can have also bad rings if you have bad compression the rings can be worn due to blowby of the pressure in the cylinder going into the crankcase, you can also have bad crank seals, the oil from your gearbox is going into the cylinder and into the combustion chamber causing your plugs to foul, i buy my top-ends from yamaha of troy they run about 89.00 for the whole kit, sounds to me like you have to replace your top-end