so i assume it broke off flush with the head?
ie:nothing to put vice grips etc on?
you can try an easy out, and if that doesn't work you can drill it and tap it, but honestly, you'd prob be better off taking the head to a machine shop right now...
things stuck in aluminum have a way of going from bad to worse REAL fucking fast, especially if you're not very experienced in this sort of thing...
it'll prob be less than 50 bucks, and done perfectly