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
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
I have 1942 tanks.