Possibly if there is enough room you could drill the spot weld from the outside and bolt it together putting a fibre washer on both sides.(use the headed end of the bolt in the tank that has the metal washer and head as one part.
If you can't braze or silver solder the tank yourself , I don't think it's worth fixing , even a tank repair putty is at least $10. you may as well spend a little more on a new tank ,or just fit any tank you can find.
