Forget about pvc parts,will not fit. To replace the flange you must rework the lead sleeve. Not too difficult if the lead is in good condition and you have watched it done a couple of times. If lead is broke and/or deterioated,it's not a good project to self teach on. If no water leaks when flushed, find a cedar wood roof shingle. Use portions that are right thickness between floor and toilet in two or more places to stop wobble. Carfuly use a box cutter or other razor sharp blade to cut shingle flush with toilet. Buy new bolt covers and caulk to finish off.

PVC parts that are designed to fit into other PVC parts will not work. However PVC parts that are designed to fit into and work with cast iron parts do exist.

http://www.oatey.com/products/drains-and-closet-flanges/closet-flanges/replacement-closet-flange

http://www.oatey.com/products/drains-and-closet-flanges/closet-flanges/twist-n-set-replacement-closet-flange

I used the first of the two products above recently, but I had to go to a plumbing supply store to get it, since Lowes didn't carry it.

But I think there is a good chance that the existing flange will not be rusted out, and you may just need new bolts. Or if the flange is rusted to the point of failure or cracked a closet flange replacement ring may do the trick.

http://www.oatey.com/products/drains-and-closet-flanges/closet-flanges/stainless-steel-closet-flange-replacement-ring