Hi,sounds like the main bearings have gone allowing the crankshaft to move from it's axis causing the gear on the primery drive to wrongly mesh onto the clutch and the piston to knock on the barrel and or head.The smoke will be the gear oil as it gets warm and thin enough to be sucked through the gearbox side main bearing/seal and then burnt in the combustion chamber.The main bearings on a gasgas incorporate the seal .
