That's an easy fix. Pull the bellhousing and get someone to steam clean it, totally grease-free. Find a 3/8" stud the right length to stick through the starter housing, with coarse threads on one end, fine on the other. Clamp the coarse-threaded end into the threads remaining in the broken-off piece, and have a very good welder braze it into place. I've fixed several of them that way, with no problems whatsoever. The stud replaces the bolt through the starter housing, and a nut and lock washer gets installed from the front side.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!