One option is just ignore it and run 4 bolts, as long as the two broken ones aren't beside each other it won't cause a problem. One of the old weight weeny tricks was just to run 3. I ran 5 for years after one of the mounting holes on my hub broke.

Drill them out and install helicoils. Lazy half assed option, drill them out and run a bolt all the way through the hole and put a bolt on it.