Hmmm...tall order...only made 11,000 of those according to Google...
Might have to consider maybe a fuel cell, or a generic tank from Jegs or similar? (Or of course having one custom made)...
There are folks that now CAD/CAM parts and hydroform them...I've seen fenders and body parts that have been done, and you could probably have someone do that (maybe 2 halves that are joined)?
Here is what I was talking about above--these guys can 3D print metal parts, as well as hydroform complex shapes (such as a fender or a fuel tank) from steel or aluminum. Video is from Bad Chad but is worth watching...gives a walk through of the shop and the kinds of things they can do:
I'd imagine it might take some work (and possibly an old fuel tank to serve as a model) but once the model is designed fabricating the part would be easy. If they can make a fender for a Duesenberg from scratch, well...
Our groundbreaking sheet metal forming technology empowers the Saltworks 3D Solutions team to sculpt sheet metal with precision, directly from a digital blueprint.
www.s3dsolutions.com
I may contact them myself to see what kind of pricing might be available for my Willys grille guard...(quotes are free).
Hope this helps--for parts that are no longer available, a 3D service like this would be my first option to consider...