Well,

You are going to have to pull the head, no matter what, unless you decide to live with it as is.

A machine shop, or, if you have access to a mill, could drop in with a two flute end mill and machine a hole in the middle of the broken stud. An end mill does not follow like a drill does. They may be able to get the stud out with an easy-out or other magic screw/bolt extractors. Depending on how much material is missing form the head, they would then be looking at welding, facing the sealing surface, and re-drilling/tapping that hole. Also, checking to see if the welding distorted the head itself and then correcting any flatness issues from the welding distortion.

All of that might cost more than just getting a replacement head.

I would bet that EB would have some good input for you on this one.

Mike