1971 Duster gas tank filler neck and gas cap

I'm looking for a filler neck and gas cap for my 71 Duster. I don't want a 70 or 72-up neck and cap as I would like to use the correct parts on my car. A previous owner mangled the lip of the neck where the cap twists on and put on an aftermarket locking gas cap which doesn't seal. I think I'm losing a lot of gas to evaporation so I would like to get this fixed.