Can I use a PVC cleanout plug in a Cast Iron Cleanout?
Another option would be to cut the pipe above and below the cleanout, and install a PVC Y-fitting with a cleanout cap using no-hub couplings like these:



I would think that drilling and/or sawing out the old cast iron cap is going to be a giant pain in the rear, and will be likely to damage the cleanout fitting to the point where it needs to be replaced anyway.

Make sure your cast iron stack is supported above before you cut it.