The stud for the bracket on the direct drive 27MT starter is actually just the long end of the really long bolt holding the starter together. Make sure the other long bolt is there and tight. Then just unscrew it, go buy a new one and screw it in. Or, remove it and then weld it up.
The gear reduction 28MT starter actually has a little aluminum ear sticking up for the bracket stud. The little stud just scrws into it. Drill it out and put in a new on.
I recommend repair rather than welding because even though you could do it right. There are a lot of ways to mess up $150 worth of starter just to fix a little $5 at most bolt. Bearings and things that turn don 't take kindly to high voltage. Especially when those things have their own windings and such.