The best way to fix it would be to tap in a 3/4" or 7/8" bolt, cut it off flush and then drill and tap a 10x1.25 mm hole for the mount. but you'd need a good drill press and a steady hand.
Or
Remove the old helicoil setup and tap it for a larger size standard bolt helicoil. If the bolt is too big for the cast iron mount drill that out. Surprisingly a cement bit works pretty good for drilling cast iron.