stripped head stud socket removal

If that's the last one you may be advised to re torque the others to hopefully reduce the load on that one.
Then find a very slightly larger spline or even a hex bit and with just one solid hit with a good say 4 lb hammer.
Having driven the bit deep you may be able to then tack weld it to the bolt.
This is not the time to fiddle fart about. You will only get the one shot at it.
It's made from very hard material so almost impossible to drill the head off it.
Someone else may have a better solution ??

Falcon, brilliant, your 4lb hammer worked!  I scratched away with a dental pick to clean out the last of the grunge and there was about 1/3 of the original splines at the bottom.  It looked like whoever torqued the head previously didn't have it fully inserted and it smeared the splines at the top.   The top of the cap was still too tight to insert the splined socket so I wondered if it might even get some bite there if I could get it fully inserted.  I taped the splines on the 1/2" socket to the head bolt socket depth, carefully lined up the 1/2" spline socket so that it was aimed at the splines at the bottom of the cap, got my 4lb hammer, and gave it a couple of whacks until it seated at the taped depth.  Got my long 1/2" socket bar and the head bolt came free!  I can't thank you enough.  Onto the next problem:-)