is the dogma frame died?

I was working on the rear-end of the dogma, creating the forks to weld on the original down tubes, when I noticed a circular scratch on the seat tube, lower than the clamp area of the derailleur.
I removed off the paint and it is a crack ... the second crack on the frame!

the first came out many years ago, and the official distributor told that the frame was out of warranty.
The crack was in the left tube of the rear stay, near to the bottom bracket box. A well-known welder close to Varese welded that crack, but this time I don't know if it's possible to repair the damage...
Anyway I need to know also if I should use aluminium or alu-mag alloy for redo the rear forks, so let's go to the welder, and fingers crossed!