Butt-Weld brazing would be weak, and would require a lot of material build-up around the joint. It could be done, but wouldn't be my choice.

Brazing an external ring or sleeve over the crack might be the strongest, and nearly as strong as a typical lugged frame, except that the tubing is thinner mid-tube.

If a good TIG welding job is done, it will weaken the metal slightly near the weld, and the metal is very thin, so it is hard to say.

I wouldn't invest a fortune on restoration in case the crack would reappear, or other stresses are found, but you might get a good rideable frame.