First, be sure it has the right bolt. It should be a 7/16-14. Be sure someone didn't put a 3/8 in it's place.
If there is any thread left at all, you could put a 7/16 bolt in from the inside, snug it down with some lock-tite, then use it as a stud. Just be sure it's threaded all the way to the head.
If you do have to drill it out and retap it to 1/2-13, you can drain some of the oil out, clean the inside with carb cleaner, lacquer thinner, etc, make a duct tape catch pouch around the inside of the hole. That will catch the chips from drilling and tapping. Also coat the drill and tap in wheel bearing grease, that will help catch chips too.