Both the Superior ring, pictured in black above, and the Pasco ring, have some holes in addition to the holes in the tab. You just have to counteract the lifting force on the new closet bolt, so you don't need to use all of the holes.
If you drill for the holes in the tabs, you would need clearance holes in tiles. What is below? You could screw into flooring. If that were too weak, you could access from below and screw into something else, or use through-bolts plus nuts and washers.
Another possibility is to drill out the old screw, and tap a new 5/16 threaded hole for a new stud. There are more possibilities. Another possiblity is to grind a slot into the side like you have at the 5:30 position. That grinding would be tricky I think, so I would not be thinking about that one as a best plan. Still, if you messed up or removed the small tile right there, you could maybe make that work.
I would buy a repair ring, place it over what you have, and see if you can get by with just t-bolts into the slots holding the repair ring down. Then cut off those fasteners above the nuts. Drilling cast iron is not supposed to be all that hard. So I am thinking maybe 1/4 inch or less, and then use some #8 deck screws through the clearance holes in the cast iron into the wood.
There is more than one right way. The toilet covers the work.[/QUOTE