Let's see a picture of the tank, so we can understand what is wrong with it. Welding the aluminium tank is impracticable, not only because aluminium welding is difficult and expensive, but also because the tank is made in 2 halves, attached to each other with a mastic seal in between. If the tank is heated that seal will melt, run out, and leave you with a useless leaky tank.
If the tank is damaged you will need to get a second-hand one to replace it. That should not be difficult. However at the moment I don't understand what could be wrong with it, unless it had received some unusually savage treatment at some point.
