Broken manifold stud in head

Use a torch to turn the extracter cherry red which removes hardness (aneils) the material. I like to use an over size drill (3/8') piloted by the manifold) and remove just a tiny bit of the head just to be sure I'm on center of the broken bolt. Irrelevent since you already drilled the bolt. A 1/4 " drill bit and a 5/16 tap and good luck.