It is a problem. I found a used one from a barn-find parts car. But during my search, I found a product to reseal the interior of lacquered old gas tanks... It sounded credible and I was ready to try it out when I found the replacement tank... If your tank is not all varnish, and just rusty on the inside try this:
If it is varnish - try this first: Berryman's B-12 Chemtool (in one gal can).... then do the resto on Youtube... search online and you'll find a lot of gas tank resto tips, especially from bikers. Hope this helps.
Jason