I suspect you can find a suitable toilet. Camping World and other RV sales/service dealers have plenty to choose from.
I don't think the flanges have changed much (if at all) over the years. Check www.thetford.com
Thetford toilets were used in many Airstreams. Do a little digging on their web site and you'll find a cross reference list Find your original/current model and see what's currently available. Be sure to consider the space required behind the toilet and space required to flip open the cover and seat.