WHat is left in the hole of the spark plug? I assume that the threads only are left in the hole. It is hard working down the hole so I would pull off the valve cover. Then I would carefully see if I can get another tool to grab the plug and turn it like a vicegrip. If not, you need to use a drill and drill the center of the plug out. Before this, go to a tractor supply store or tool store and get an EZ out. It fits in to a hole and turns in the direction needed to remove most bolts. Read the package for the drill bit size to use and some ezout tools come with the right drill bit. You put a wrench on the ezout and turn out the tip of the plug.