You know I was wondering when someone was going to ask this. You can get hurt or killed fixing a fuel tank.
Here's a product in the J&P catalog 1-800-397-4844 called Kreem #702-469 and sells for $29.99 it will repair the leak. you can also go to Parts For Your Harley®, Honda Motorcycle Parts - J&P Cycles®
that's the safe way to do it.
Now the dangerous way. Empty the tank and flush it wit hot soapy water and I mean lots air out and do it again at least 3 times. Seal up the tank and put it in a tub of water just like you do when looking for a hole in your tire. When you find the leaking spot mark it so you will know where it is. Clean the area good with sand paper do this by hand don't want any sparks and to bare metal. Now of the part that can really get you hurt. Small leaks can be soldered with the type of solder that plumbers use and the use of a propane torch that heats up one of those large soldering irons. Or by direct heat from the torch. And you can also oxy weld it. The most important thing is the use of an inert gas in the tank while doing this and that's the catch. The use of exhaust gas works where you afix a hose to the cars exhaust and stick the other end in the tank you are working on or the use of nitrogen purging in the tank while you are doing the work. You see there's a bunch of ways things can go wrong and it will or can burn you or blow up and kill you and others. If I didn't scare you enough not to do then good luck. I've been involved i tank repairs for many years 1000 bbl tanks and tons of training and we use very special tools that the average Joe doesn't have. So there you go a safe way and a dangerous way.
the address of jpcycles is in the edit box.