Might be an exception, but all of the flanges I've seen have either a single slot where you slide the bolt in from the side, or a T-slot, where you have more flexibility on where to mount it. But, with the flange below the finished floor, you may not be able to slide it out, even if it did exist. A good wire brushing may reveal it. Do not use steel bolts again! Use brass or SS, not brass plated steel or plain steel. They make flange extension rings and flange repair rings. One or the other or a combination of both would get you a way to anchor new bolts to the existing flange. Using an internal flange is not the best choice, especially on an older pipe with corrosion, especially if it is a 3" pipe (4" does work, though).