I'm not sure if you can easily disassemble the gas cap to replace the cracked/crazed original center. One thing you might try: I've improved the appearance of some by applying a gloss paint-on clearcoat polyurethane. It won't repair deep cracks, but will kinda disguise surface scratches.
If that does not work, the good quality repros are good. Dickson
1965 GT fastback
There's a fine line between "hobby" and "mental illness" - Dave Barry