I would feel much better if I was able to remove the stud and reuse the threads. Grind two flats on it to get a good vise grip on it, heat the stud red hot with the vise grips off, then clamp down and swivel back and forth to work it loose. Red Hot, remember Red Hot, especially the back side you won't be clamping down on. You will be able to do it with a propane torch, especially because there is not much metal mass around it. Remember to use lots of layers of aluminum foil around it to protect the rest.
When I had initially mentioned manifold bolts, I was referring to the bolts that hold the manifold to the engine. Check them to make sure the bolt heads are not missing. Look for burnt & leaking gaskets. If all looks good, do NOT attempt to tighten the bolts. Leave well enough alone. You don't want to break one.
But it was surely a good thing you found this leak.
