There is a method where you weld a nut of appropriate size to the stud stub.
This is easier if the engine is on a stand...and you need a welder
You hold the nut in place and (I used a mig welder) start the weld on the stud, take your time using circular motion fill the threaded center of the nut and at the same time attaching it to the remaining stud...you can do this even if the stud is broken off flush. Use plenty of heat.
Let it cool a little then use a wrench on the nut to loosen the stud. Sometimes a rattle gun on low is better than a wrench.
It may not work the first try, some of the head studs I removed this way required three or four tries, but it eventually worked.
I'll post pics if i can find them..
can't find the pics, but this gives you the idea..
https://www.youtube.com/watch?v=aRrz-cphBY4Randy