Seems to me you have two choices. One is to remove the old cast iron flange, possibly by tearing things up to get the old screws out and breaking the connection to the soil pipe or cleaning up what you can and simply adding a steel ring on top of the cast iron. The steel ring is fastened with screws to the floor and has built in keyhole slots for the toilet bolts. This assumes you have solid subfloor to hold the new screws. If you've stopped the source of water that caused the rust in the first place, the steel should be OK. If everything goes well, the new ring should work just fine. Bob.