what to do : i suppose you have ignition, and it is ok,you changed spark plug by the original spark plug,then fuel i suppose is ok, does it flow after the valve ?, if so, then do you have enough compression ? is the top cylinder not leaking by the joint ? try out the stop on the fuel tank is the small hole ok ?
try also the following which helped me also a lot
look out for a place where no houses are and detach the exhaust from the cylinder, put stopper in your ears and let the motor run, if it stays running the exhaust is plugged and best thing to do is by another one
hope this helps