Get a service manual (clymer or factory) and do it right. I would never hammer the piston out unless I was planning on replacing the cylinder and crankshaft, which I've done a few times. Find out why it is hanging, could be trash in the cylinder or something jammed in the crankshaft, could be the piston ring tip is hung in a port, could be siezed and melted to the walls. Remove the head/pipe/intake port and get a better look at the issue. If all is well, I would loosen the cylinder base and slowly raise it while lovingly tap the the piston out with a rubber hammer and maybe a wooden block. Some might have better ideas. But I would only do this knowing I am sending the motor off for a bigbore, which I always do on a lockup. Why not, it is the same price as rebuilding and plating.
