40 yr old cast closet flange has deteriorated.

I should have replaced it when I put tile down 6 yrs ago. It holds the toilet bolts fine, but the drain opening seems to have enlarged to the point where the wax ring slips into to drain instead of compressing onto it. Aside from trying to chisel this flange free and ... Its set in 6 inches of reinforced concrete topped with terrazo and a layer of multipurpose thinset with porcelain tile. Ugg. I ahve a jackhammer, but I dont want to disturb the ceramic if at all avoidable. Its my home. I'm not a plumber, therefore I can legally experiment. The flange is flush with the tile, so it needs raising anyway. That said, can I try something creative like grinding the rust off and using epoxy to secure a flange spacer to the cast iron? I will utilize the anchoring capacity of the old flange to secure the toilet, the spacer is only to provide a base for the wax ring to compress and seat upon. The big question here is what will effectively seal pvc to cast iron in a non-stress non-pressure environment. A-B epoxy? Or do they make a flange that will slip inside of my existing pipe and be secured/sealed with soldering? I'm afraid to try that option myself with the tile there and moisture pockets and the dificulty level of a big sweat job. Maybe I can grind it smooth and obtain a cast iron flange spacer and have it welded to the old flange? Or a smaller diameter pvc pipe (3 1/2") and 4-5" long with flange that I slip into the cast drain, and surround with silicone, let it set, and use the old iron flange as a secure anchoring point for the toilet bolts. My goal is obviously to eliminate the seepage from around the toilet when its flushed, and try a shortcut before a big messy job.

They do make a flange that will slip into your pipe. Go to L*wes and they will have one. There is also one that slips into the pipe and with an allen wrench you expand a gasget against the inside of your pipe.

CI mechanical flange?

After perusing posts here in the forum, I discovered a similar situation. and pictured was a pvc flange with stem and seal that goes into the cast pipe- BRAVO!. I'm glad this was invented to fix the problem of cast flanges gone to rust and the skill level required to replace them exceeding that of joe average.
so to secure this device do I :
1) put new bolts through my existing flange and the new one with nuts to lock it all together (existing flange is not broken, just corroded opening and its low)
2) remove cast flange and set the new one in concrete (to lift it to proper height), let it cure then use tapcons to anchor.
3)attempt to drill holes through the CI flange so I can anchor the repair flange/stem pvc w/seal part to the concrete below it