Been there tried these things.
Your really better to cut out the area with a sawzall and use rubber
ferncos to join the PVC with the old cast iron pipe.
Do note this area will conduct the sound of water going thru drain
If noise matters you can use cast iron thats designed to be joined by
rubber ferncos, it costs more but will be quiet.
if theres a lot of cast iron over head add some steel strapping to
support the weight before cutting out the bad area, although it usually
doesnt matter, but better safe than sorry