Cut a plastic milk or water bottle into small pieces, and put them in a can. Tuna cans work very well for this. Build a small fire to melt the pieces to almost boiling and pour it over the crack in the fuel tank, forcing the hot, gooey stuff in as far as you can. Smooth it out with a spatula or tire iron. When it cools it will have a good, strong seal and last for a long time. You can also fix cracked battery casings the same way.