I recently went through that with my Dakota pickup. I found a peace of steel tubing that fit snuggly into the manifold hole (with a little electrical tape wrapped around the outside of the tubing to make it fit tight). I cut it about 1" to 1-1/2" in length. Then, looking down the center of the tube with a mirror and light, I could see that it was well centered on the broken stud. Then using a right-angle drill motor and a bit that was large enough to not wobble inside the tubing, yet small enough to not exceed the minor diameter of the stud threads (or least not by too much) I SLOWLY drilled down through the center of the remaining stud. Using a combination of a tap (original diameter and pitch), a magnet, an awl to dig out small curly pieces of old threads, and blowing it out with compressed air, I was able to chase the remaining threads out the hole and insert a new OEM stud.

Patience is the key.
--
Chris - 1990 740, 1973 1800ES (Non-running), 1993 Dodge Dakota (Still has a little paint left on it)