Pull the plugs out, fill each cylinder with Marvel Mystery Oil. Let it set several weeks. Take the tin flywheel cover off the bottom. Try to turn the engine each way by using a pry bar on the starter ring gear. If that doesn't work, consider removing the head. Use something like a 2x4 on top of the pistons and hit each one several times, and try turning the flywheel again. If this doesn't work, get a bigger hammer. You will probably need to bore the engine anyway.