ok then its a problem inside the gearbox at the very back of the engine,if you have the cover off and turn the back wheel with your hand does the clutch bell spin?if not then you need to take the transmission apart to see whats up inside it,i expect the helical cut gears are stripped of teeth ,before you go spending money on bits make sure the holes that the shafts sit in on the engine and the cover are not worn oval as if they are you need another engine for it.
adam bike doctor
July 2010