Depending on what's around it and what size the existing port is, and how unlikely it would be to find another, better tank (IDK anything about your specific tractor or how available parts, either new or used, are for it), you might consider buying another tank with a similar sized port that is rust free, and then cut it would and braze/weld it to the original tank.
You might also search your area for people/companies in your area that part out old tractors. They might have the correct tank for your tractor, or at least another tank that you could use to make your tank work.