If you use the hammer and chisel approach, make sure you are striking the chisel at an angle in the loosening direction around the largest diameter area possible and do not contact the engine casing and damage the mating surface. There is lots of material there to work with so you should be ok. Take a file and make an engagement point to work the punch against if you can't get a good bite in the plug with the punch alone. Use an all steel punch, no wood handle and a framing hammer or one of relatively similar size to carry the impact to the plug. Once you get it out, trash the plug.
Good luck!