G'day have unseized a few engines over the years, although they had cast iron pistons.
the best method i have found is soaking in automatic transmission fluid for around a year and then hitting piston out with a sledge hammer and a block of wood.
in your case you might damage the piston, however if you shaped the end of the piece of wood to be a snug fit in the bore and hollow out the end of the piece of wood so that the force is directed to the outer part of the piston (strongest). Set the barrel up on blocks with the conrod hanging down and then give the wood a good strong hit with a heavy hammer. If you can it to move a fraction the battle is won and would tap out fairly easily. Another thing that helps too is heat up the barrel etc in you bbq, take it out and poor some icy water into the underside of the piston to shrink the piston a bit. hope that makes sense.
