Given that it's 6061-T6 alloy (not 7005/7020), a fully correct repair would require major solution and ageing heat cycles to get the material back to full strength, so pretty expensive process (several hundred £s...).
However, the front mech hanger is not a majorly stressed or structural joint, with the added benefit of a fairly thick butt above the BB to mount it to.
Using a non-heat treatable but higher strength filler, such as 5183 (rather than the normal 4043), would give a decent strength for a welded, but non-treated repair here.
Alternatively, a new hanger could be aluminium soldered/brazed on, with the correct filler and procedure. It's not too common, usually rejected in favour of faster and cheaper welding, but gives a good join with far less heat and stress raisers than welding.
It's not a repair that I have done before, (mostly headtubes and dropouts), but I do have a stock of 6061 hangers designed for welding (or solder/brazing), and also cast stainless and aluminium hangers designed for riveting on.
All the best,