Jeff we need more information but I suspect any derailleur would work and will cost more than the bike will ever be worth. I feel you would be better off and learn more just trying to clean up and fix what you have. Rust can be removed and oil can improve things. Take the bike apart and just start cleaning and oiling things. Possibly replace the cables. You can buy enough cables for that bike at Walmart for about $10. Beyond that I wouldn't put anything into the bike. Roger