When trying to drive the piston out, alternate the hits between the top and bottom of the piston. the idea is to get it loose first. If you try to hammer it out in one direction only, your chance of success is less.
To gauge piston movement, put a scratch on the piston thru the exhaust port with a sharp instrument. The line should be close to the edge of the port so you can see it's movement.
Keep adding WD-40 or Liquid Wrench as needed and take your time.