No clue on tank replacement but ....might you be able to put a liner in or seal the tank so no more rust gets into your gas line? What I'm not sure of is if you would have to remove the tank to apply this stuff ..but here are a couple links I found. I have not tried this stuff ...just searched and found it as I had read of a car restore that did this on his rusted tank. If I can find that tidbit again I'll share it here but so far I have not been able to locate that one.
http://www.eastwood.com/ew-gas-tank-sealer-kits-for-cars.html
https://www.kbs-coatings.com/tank-sealer.html
http://www.eastwood.com/ew-gas-tank-sealer-kits-for-cars.html
https://www.kbs-coatings.com/tank-sealer.html