Toss it. A leaky tank will cause you nothing but grief. It's easy to replace a tank. If you've got an ARF, the odds of finding the exact same tank on your local hobby shop shelves are slim. ARF tanks are rarely the same as retail tanks. The ones you'll buy locally will have their capacity in fluid ounces, while ARF tanks are almost always Metric. ARF tanks are generally lower quality, too.

All you need to do is find something of about the same capacity with similar dimensions. It will fit, and it won't be tough to do. It may not line up with a firewall hole, but that's no big deal.