I don't think welding is the answer for several reasons.

Better solution IMO would be to get a strip of 6061 (maybe .090 x 1/2"?), bend it in a vise or whatever, notch out the arc for the bar, and JB weld it inside the lever body.

BTW if you're fishing for un cracked Universal brake levers, I think I have a pretty minty pair in my junk box I'm never going to use.