One used to be able to buy just the inside tank, and I have done that a couple of times, years ago.
The mystery to me, is that the tanks were made of aluminum, so they should not rust.
Someone might have used some galvanized fittings on it, and they will rust and cause yucky brown water.
The only reason that I had to replace tanks was that the chemicals in hard water ate through them, and they became holy (wholey) and righteous.
Have you tried really flushing it out, preferably with some vinegar, and replacing any galvanized fittings with brass or pvc ?