cleaning out the fuel tank and fill tube

I've used kits from KBS to "restore" motorcycle tanks which, as 67Dart273 mentioned, consist of a degreaser, an acid to remove/treat rust, and a coating to seal it all up. Takes time and patience. If it's in your budget, buy a new tank. The filler tube can probably be cleaned up.

http://www.kbs-coatings.com/tank-sealers.html