A new copper spark plug gasket might conform enough to seal the leak, but you would have to replace the gasket each time you removed the plug. It could be silver soldered IF you can get all the oil out of the cylinder casting. (Plus some very careful grinding to make it smooth.) With a heavy duty drill press and a BIG drill bit you could deepen the hole and use a thicker copper washer under the plug. OR just keep cleaning up the mess. OR find a good used cylinder. . .
