Last one I had to do in a aluminum cylinder head was easy-goes like this:
Thread on a nut to WHATEVER is left-no matter how little.
Then, take an arc welder and weld the center of the nut to the remnant of the stud. This gives you something to hold onto, and conducts the heat down the stud, expanding it in the aluminum, and as it contracts, it breaks free of the aluminum-out it comes!
Works better than any removal bit I've ever used-even with the head locked in a vertical mill!