I've tried it with Evap-O-Rust on my Honda 90, worked fantastic! Simply add the empty tank, leave it overnight with the cap on, pour it out the next day (it's re-usable, save it). After it's drained you can rinse it out with solvent and seal it with gas tank sealer, or if it's solid with no leaks just add gas and go.

By the way, from thier website:

Why Use Evapo-Rust ™� Removes rust quickly without scrubbing.
� One gallon de-rusts up to 300 lbs of heavily rusted steel.
� Removes even heavy rust .. COMPLETELY.
� No fumes or bad odors.
� Non-toxic. Safe on skin. Non-corrosive.
� Non-flammable. No VOC's.
� Easy to use, immerse part in solution. Water soluable.
� Requires No Special Equipment.
� Contains no acids, bases, solvents or other toxic ingredients.
� Can be disposed of safely into sewer.
� Will not attack un-rusted steel.
� Safe on other metals.