DIY
I'd start with a quart of lacquer thinner or acetone or some other toxic nastyness that will dissolve the varnish.
Screw on an outlet tube in place of your fuel tap, then cap it off. Throw in a handful of nuts and bolts and slosh it around for a while until you think the varnish is cleaned off. Drain it then dry it out with the hose of your shop vac on the outlet to blow air in the tank.
Next step is for the rust. This time use a quart of Evapo-Rust and even more nuts and bolts. It works best when warm, so do it inside your heated garage. It takes a long time to work, and you have to keep sloshing the juice around to keep the surfaces wet with fresh solution. If you do it on a day off when you're doing other things around the house, you can go out and mix it around every 15-20 min.
gsun, do you still have the instructions from Mark?