Replacing toilet but running into flange problems

[stylistown]

Hi Everyone! We are new to airstreams, just purchased a 1963 Globetrotter and trying to upgrade the toilet to porcelain. The flange / bolts needed to be spinned in order to fit the toilet, however it broke in the process. We have been searching high and low for a replacement Bolt Closet Flange Socket but so far nothing fits. 4" is too big, 3" is too small and won't seal. The opening is 3 1/4" and it seems like it goes into an old metal holding tank. Any advice on how to make this work?