I recently had a few leaks develop in the metal coolant reservoir on a 1999 Freightliner...one at a seam and two where the top corroded right through. I ordered a replacement tank but it had a week lead time and I needed to leave the following day. So I went to O'Reilly Auto Parts and bought a can of Red Kote fuel tank sealant. It's similar to wing tank sealant used in aircraft.
I cleaned the inside of the tank with some acetone and smeared some JB Weld on the outside of the corroded holes. I then swirled some Red Kote around inside. I drained that and let it dry, then did that one more time. I only used 1/2 of the can for both coatings.
The reservoir worked fine and was leak-free for the entire HOT trip. And...I am kind of embarrassed to admit...I'm still driving with that reservoir about 2 months later. It's still leak free, and I have the new reservoir in the truck in case the original one failed, but it has held up so well I haven't gotten around to changing it at home. I should do that, though. What do they say about the cobbler's kids not having shoes?
My point is that it would definitely be worth a try, if you can't find a replacement tank.
Damon Industries Red-Kote Gas Tank Liner