Hi, I've got gas leaking from the gas cap when tank is near full. I suspect the 51 year old gas cap gasket has hardened and is no longer sealing and I don't want to keep the tank only half full. Has anyone used one of these e-bay reproduction gas cap seals (https://www.ebay.com/itm/275162293539?var=0&mkevt=1&mkcid=1&mkrid=711-53200-19255-0&campid=5338590836&toolid=10044&customid=61d11ae9f5701daecbff7c749e37328f).  If so how well did it work? Thanks.

E85 compatible?

No, I only run alcohol free gas in the Z.

The seller should tell what the material is.  Might be fine, can't tell.

Here's a different option. 

https://www.mcmaster.com/products/rubber-gaskets/o-rings-1~/square-profile-oil-resistant-buna-n-o-rings/

Based on the reputation of the seller, I would say good to go.

UPDATE:  I bought and installed the reproduction seal from the ebay seller (link in my original post). Fit perfectly and so far no gas leaking from the filler cap. I am quite satisfied.