For chainsaws there is a gadget for locking the piston that screws in place of the spark.plug and basically is just an extended long piece of rod with a spark plug thread. You could try something similar to lock the J.A.P. and put your screwdriver through all those holes and give it a sharp tap with a hammer. Make sure the piston is hard up against the rod before you do though. Probably be good to hit it with heaps of penetrating oil overnight first as well.
