I would think you could make a gasket. IIRC some of the old ones were nothing more than a cardboard like material. As long as the small vent hole is not covered a home made composition or cork like replacement should work.
51-6 gas caps are the same and from
this old post it appears the old Stant number was G-37. That cap was apparently fairly common at the time and was used on several cars of the era.
Here is a current ebay listing for the old G-37 number. The new plastic replacement for a Stant G37 appears to be a completely different cap but since many of the references say it replaces the G37 it might fit the tank. If not, and no one has a current number, maybe you can cross at the parts store by finding a listing for a Chevy or one of the other cars mentioned by various vendors as using the same cap.
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