If you have a set of SHARP drill bits and a steady hand I would be inclined to drill the head off the stem bolt - as long as you're sure it's just a rounded bolt socket.
If the stem is stuck then cut the stem off the quill so you can undo the headset then heat the quill, trap it in the vice and fit an old front wheel to the fork for better leverage as you twist the fork off the quill. Appreciate the stem's toast . . .