These are potmetal and can't be effectively welded. The best you can do is use a high strength catalized plastic like JB metal repair or the like. Still, if the part is seriously cracked the best solution is replacement.
I used fiberglass and fiber cloth to repair some damage on my right headlight housing. All repair work was on the backside so it wouldn't show. It seems quite strong. I dropped the whole thing on a concrete floor and redamaged it. My previous repairs held just fine. My car hasn't been on the road, so I don't know how well the repairs will hold up to extreme temperatures, vibration, and normal body flexing.
I suspect the only really permanent fix is replacement.
I was going to try leading one to see how it will holdup but haven't gotten around to it yet.However on one of the extra's I have for the 69 there was some bondo repair done.This car has been sitting in a scrap yard for the last 15 years and there was no sign that the extention had been fixed. Never noticed it until I cleaned the paint off of it. Don't know why lead will not work, just a bit touchy to use, take all safety measures , mask etc, if you try this .