The tanks are made of high density poly ethylene (HDPE). I've repaied several with a plastic welder. You can get a welder at Harbor Freight. If you need to add material, make sure it is also HPDE, or it will not work. Sometimes you can cut some off a seam to get the extra you need.
If you look around, you can find kits for repair Coleman canoes. Coleman call their material RamX, but it is in fact HPDE. Those kits will work for your tank.