You can drill out the hole, anodize, then get the head re bushed with brass, thrd'd for the glow plug. the new bushing is stepped, flange on the combustion side and seated in a counter bore for the flange. works great. Good project for the fledging machinists in this group Done right, they are much better than new. I wish engines made these days would go back to the brass glow plug holes. Last a lot longer and harder to strip.