The only sure way is to cut it open and repair the affected joint between the two tanks. I think it's just a welded in wall. It may have come loose or rotted through. Then you have to weld the whole thing up again ensuring its leak proof. As it happens I have a Cometa tank for sale! PM me if interested.
I should think a low temperature brass rod brazed repair would be preferable . Less heat distortion and less chance of blowing holes I should have thought .
I've just done a low temperature Aluminium brazed repair to my crankcase with an ordinary blow lamp at just 300 degrees