It probably needed to be replaced anyway so don't sweat it, 50+ years old!
Melt the lead off of the ferrule, (the wipe joint near the cast iron hub) and install a transition coupling. Fernco Pro-Flex is a good quality coupling for this purpose. Make sure you wipe all the lead from the brass ferrule. Make the transition to PVC with the appropriate size coupling. Don't forget to anchor the new closet flange to the floor.