I am in the middle of a "project" to make such a repair. A whole arm assembly was more than 50% of a new awning. Careful examination shows that the arms are made out of extremely thin aluminum channel extrusion. The one that got "mangled" the worst is 60mm wide (just under 2.5") by 50mm tall (about 2").
I bought a piece of 2.5", 0.125 square tubing and cut one face off, making it a channel,but it is still too wide by about 4-6mm (about 1/4"). Next plan is to narrow the channel by taking 1/4" out of the middle of the 2.5" face and welding the 2 angles back together. (If I was starting over, I would just buy 2.0" 0.125 angle and save a few dollars, although one side of the angle would have to be trimmed to 1.25")