i used this really cool stud extractor today that was like a drill chuck, but with more sharper points of contact, and it worked in reverse, so you could undo it. I was skeptical, but it came straight out, and it only had about 6mm of metal to grip to. no use in your situation tho.
id just cut the stud off, then drill it out, tap a new thread and put in a bigger stud. you could do it yourself, and would probably only take an hour. on my mini i did this with a bendy extension attachment on my dremel, the bolt sheared off into the block, and to avoid taking the engine out, i drilled out the bolt and re-tapped it. it took me 4 hours tho! i wouldn't recommend that.