I've cleaned a few by pouring old nuts in them and then shaking them for a while. A long while. Gravel will work too, but you can get the nuts back out with a magnet.
If you really want it to last a while, and remember, anything you do to established rust has a limited life expectancy, you need to coat the inside with Red coat or some such fuel tank sealer. The thing is, that a quart of Red Coat probably will cost about what you gave for the tractor. (Which was a steal if it didn't take much to get it to run.)