Personally, I would probably do what others have suggested which is to try to drill out the old screw. Grind the frozen stud down to a flat surface. Center punch the stud- do a light punch first to insure that you're on center, then re-punch for a more defined dent. Get a good set of sharp drills and drill it out. Re-cut the threads or go to the next size up if necessary.

If that doesn't work, you might consider going to a weld/machine shop and having a custom gugeon made up with a new bolt pattern. I would look into the cost of that before monkeying around with cutting holes in the boat etc.

sm