I would prep the plug surface by scuffing it to expose bare metal. Get a nut that is pretty good size as you will weld the center or alternatively since it is a hex key get a bolt that is about that size insert in the hole which will naturally center the bolt. Then I usually use a gas mig welder and get a good could beads on it. The heat is also going to help you here. It should come off with ease. Just make sure you have a new plug on hand to replace it with.