Progress thus far:
The tank smelt of old gas so I let the interior soak for a day with Acetone, with occasional sloshing. This removed the gasoline residue and revealed a couple of places on the bottom, near the front of the tank where (apparently) old gas & water had collected and the factory tank coating had failed and lifted.
The next step was to agitate the tank with a pound of drywall screws inside. Manually. My arms and shoulders are still sore.
This scuffed the interior of the tank and removed any loose tank coating. The only spots where loose coating was removed were at the two low areas in the front and along sharp edges of the welded seams.
The last step in cleaning is to soak the tank in a paint stripper containing Methylene Chloride for a couple of days, with occasional sloshing. This is strong stuff, but the factory liner seems to be holding up thus far-- it's pretty tough. I'm not expecting the factory coating to come off. Then rinse with acetone, skuff with drywall screws and rinse thoroughly again. Then let dry for a couple of days. However, being Winter, I may wrap it in a electric blanket for a day or so and call it dry.
Since the factory coating is still largely intact and since wither of the one-component urethane tank sealers are rated to go over old tank liner, I plan to use the Caswell Plating epoxy tank liner which is a two-component mix that can be used over old tank liner.
Then wait for the next warm spell, apply the tank liner and let it cure for several days.
I have new gas cap leather seals and Karcoma right-angle petcocks, with filter socks, on the way and I should be in business.