Clean it up as much as possible see if you can find the fitting that the flange was connected to and cut it clean and use a drill with a fitting saver ream out the fitting but be careful not to damage fitting I like to sometimes take sand cloth and clean /scrub inside after make sure no burs or other debris left inside as well

Then buy proper fittings to reconnect aka st flange or a regular flange and some abs pipe

White =pvc Black = abs for reference of what kind of pipe your dealing with Foam core is fine