I coated a tank in the past with a product from these peopleIf it is totally rusted a problem is the rust will have thinned the metal and possibly produced pin holes in the tank.
I use critic acid to clean rusted parts but the residue needs scrubbed off the metal. That will be difficult inside a closed tank.
I'd be looking at rust converters, then tank internal sealers and internal coating products? I have no experience of these products, but I think that's where I'd start.