The only thing I can think to do is put it back together with some penetrating oil in the wells. Maybe if it's run a while the repeated heating & cooling can let some penetrant seep in. I can't deal with a broke off spark plug in the head. I'm done with it if that happens. They may all brake before coming lose. I may just leave them alone. I hope I've not done damage already. The plugs may last the life of the car. They don't look old from the top.