Have you considered this?
Caswell Inc. - Epoxy Gas Tank Sealer
Not only seals gas tanks, but can be used as a potting compound (waterproofing magnetos)
Creating non-skid surfaces (foot pegs and friction drive rollers)
More info on epoxies:
I have used it to successfully restore a gas tank for a 1965 Toro FLYMO.
The tank has so many pinhole leaks that it could have been used as a pasta strainer, but now it is perfect and the mower is used weekly.