Reweld the tank. Stainless is an excellent material for water (and fuel) tanks. The water picks up no taste as plastic and fibreglass can, you won't get Alzheimers as with aluminum
, and it's very long lasting as jrd and others have said. Every food processing plant, brewery, soft drink plant or dairy is stainless from one end to the other. After all, the tank did last 30 years! Any welder working with stainless can weld it and test it easily. Once welded and pressure tested you should be good for another 30 or so. Does it leak on the bottom? If so it might be from sitting in water.
I will be replacing a leaking molded poly tank with custom stainless just as I have my fuel tank.