How about the right size Heli-coil. Drill for Helicoil and then install the Helicoil for same size as standard glow plug. NO 2 Method: If you remove head...can you just Heli-arc the spark plug hole shut...dress-up weld in dome and drill , tap and spot face for glow plug. If that is not possible I have one more idea. Leave spark plug in and drill from inside the cumbustion chamber to a new spot where you can spot-face and tap for a glow plug in different location. Best Regards Capt,n