So it seems the premature (40 year) failure of this rear derailleur was due to an extenuating circumstance. You have plenty of options now, virtually anything you can hang there with a claw, but I wouldn't hesitate to install another Prestige (this time with some stronger (Suntour) pulley wheels).

I think Simplex rear plastic derailleurs suffer by association with the Simplex front plastic derailleurs, which truly are the bane of man's existence on this planet.