Can you see main line where the toilet flange/pipe run into? Is it easily accessible so you can cut the flange/elbow and a short bit of line completely out and then splice in new stuff?

They also make a cutter than you can chuck up in your drill that will cut the PVC from inside the flange, but if your's is indeed 1 piece I cant imagine after removing the flange only the OD or ID of what's left would hook right upto standard fittings-it might/should, but it's hard to say without seeing it