I’ve welded up a crack similar to this, where it ran in to the braze.
Just removed everything to see the whole crack and tig welded as much as I could, starting a weld pool before the crack and running it in to the crack with a filler rod. Ran it as close to the braze as possible then filled the rest up with bronze rod.
This was on a Brompton but a similar situation.
I’d forget mig on that unless you’ve done thin sheet work with a mig, it would probably just blow a hole in the tube and you would still have the braze to contend with.