Whats the value of the frame ?
Any sentiment attached to it ?
Whats the replacement value of a new frame ?
Is a fix cheaper than replacement ?
Alu needs heat treating after welding for full strength.
From mountain bikers i know that have had smaller cracks welded up it can be very hit and miss.
The welder had reccomended that the frame is allowed to age harden, you leave it for a few weeks at least.
The ones that didn't had the frame/swingarm fail again.
A crash bad enough to crack the frame like yours i would say time to retire the frame.