as m says compression is the probable cause,the head gasket consists of a rubber ring with four orings where the studs are ,a genuine gasket will last for a fair while but still end up turning to bubble gum and an aftermarket gasket will turn to bubble gum after a few weeks,check this first and after that go for the piston ,these bikes also suffer from water pump seals and cook the top end once it has lost all the water i would also suggest removing the thermostat if you do have the head off its scewed inside with two little philips screws.
adam bike doctor, May 2010
