You actually stripped the button head not the cooling head.All button head are removable from the cooling head. But i did once had a vz-b button head that seems to be Welded on to the cooling head. lol

Best bet is to get a new button head. There is no succesful method to re-thread a button head as glow plugs only comes in one size. it ends up better to buy a new button head.

On an emergency i once used super glue and gasket sealant that was good for another hour on the track then the button head had to be dicthed for a newer one .