I have no experience with this, so just wanted to make a suggestion, no idea if it would work. I don’t know if the porcelain middle of the plug is still in the part that has stayed in the head. If it has I would take a drift and try to tap it into the cylinder, this will just leave the threaded steel body, then I would take the largest easy-out that will fit in the middle of the steel spark plug body and then try turning the easy out to see if it will bite into the plug body and unscrew it from the head. If it does then you need to find a way to get the porcelain part out from inside the cylinder via the spark plug hole. (I would use a rubber tube fitted to a vaccum cleaner and try sucking the porcelain parts out). Just an idea, but I can’t think of any other way except removing the head, but that’s a lot of work and expense.