If you wash your old tank out with really hot water and detergent, you can coat the inside of it with the resin used for fiberglass. Naturally, you will have to protect the fuel outlet from getting plugged with resin but just mix up a coffee cup of resin, add the hardener, blend it and then pour it in the tank. With a strong light shining into the tank, roll the tank from side to side and around and around to get the resin to coat the entire bottom and sides. Keep moving the tank until the resin begins to harden and won't flow anymore. Then stop and leave the tank for the next day. At this point..... what do you have to lose?