I would pull the cylinder (four nuts on the head, two on the intake and the one remaining on the exhaust)
Slide the cylinder off so that you can see what you're doing. Buy a small left handed drill and try drilling in the center. Sometimes that works to turn a broken stud out. Use a drill press if you have one. Try an easy out. If that doesn't work, heat the metal around it with a torch and touch it with a crayon to melt some wax into the threads. That helps things lube up a little. Try the easy out again.
If all else fails, drill the rest of the stud out and retap the hole for a new stud.
Before you reassemble, go buy a torque wrench. It might seem a little pointless to spend so much money on a tool that you don't absolutely need, but it'll save you a lot of hassle in the long run.