There is a limit to what the epoxy sealers will do ,my Morini tank was so bad I stopped counting pin holes at 60!, I got a replacement tank.
A pricey option might be the tank man, or a similar company in the midlands whose name evades my memory at the moment, I'm sure a net search would bring them up, as has been suggested. Could it be lead filled first then coated with epoxy as a belt and braces solution?.
Alan.