If there is still metal left, may be able to clean and seal again. I have had good luck with the Eastwood 3 step kit (detergent, acid, acetone, sealer) for rare tanks. Kinda messy but works well.

Though personally I would get it out of the cab by adopting a later model tank for all the reasons mentioned. I'm pretty sure brand F and C in cab tanks are hard to find or unavailable because of liability fears.