I've never seen a cast iron tank except as a part of the
hull of a ship built before 1930. It would be a tough job to cast a tank. I think you probably have a mild
steel tank.
Recently there was a thread about a
fiberglass product you could put in a void and expand to fit. If entry is large enough you could clean the tank and
fiberglass and maybe be successful.
Usually your
steel tanks rust and leak on the bottom because of
water and organisms eat at the steel. A product of partially filled tanks and no biocide. There are tank coating epoxies made for inside a
diesel tank. The tank would have to be
oil free.
I have 1942 tanks.