First of all it sounds like you have no vent on the toilet, be sure to vent the plbg. when you change over to PVC. Send me drawing of your fix layout and if it is 1 story or 2 story, if your toilet is sitting right on pipe and no other pipes running off then you are not vented. [email protected]
Take a sawsall and get blade that cuts castiron and it will cut easy 12″ blade, and use a nohub band to reconnect. Like I say main thing is vent.
Art retired plbg