This is of little help now that your tanks rusty, but in the future you could try this. To keep your tank from rusting when you put it to bed each winter try draining the gas tank and spray or pour WD-40 in the tank. Get it lined with WD-40 and you should be good. When spring comes just add your gas and the WD-40 will mix with the gas and have no harmful effects to the running of your engine. I also spray WD-40 into the spark plug hole to keep the rings and piston from rusting.
If it ain't broke, you're not trying hard enough.
Lamborn's Miniature Engines