If your tractor is old you might try Yesterdays Tractor.
Years ago I needed a gas tank for my Ford tractor but was unwilling to fork out the bucks for a new one and couldn't find a used one so I welded my own. The only down side was that I had to settle for a 9 gallon tank instead of a 14. That was in the late 80s and I am very satisfied with my decision.
Good luck whatever you decide.
/ find and purchase gas tank (new/used) for ford lgt-145
look up POR 15 paint over rust, they have a repair for old rusty leaking gas tanks. First pour in an acid to remove rust and scale, then after that is emptied a neutralizer goes in. Finally a 2 part epoxy mix is put in and rotated around the tank. it is poured out before it hardens and allowed to dry. My old Ford Jubilee replacement tank was over $300 this fixed it over 10 years ago still no leaks!