How big is the hole? Yes you can patch a fuel tank if the hole is small. Considering it's on top, it should be a piece of cake to patch. You can use a fiberglass epoxy kit, basically like a bondo with a fiberglass cloth, to patch something small. A bigger hole I would drain the tank, take it off the car. Let it sit for a couple of days to evaporate any fumes that are left. Fill the tank to the top with water until it's overflowing, dump out the water. Take the tank to a local welding shop, tell them you drained it & water washed it, and that you want the hold welded shut. After you get the tank back, there is a product you can pour in the tank similar to a POR15 that'll bond to the metal and keep moisture off of the tank. BUT, if this is a rust hole, the tank is probably not long for the world anyway and you might start budgeting for a new one.