I would check around welding shops locally in your area for an experienced welder who could laser weld the hammer for you....then file and refinish as necessary to match the right hammer....JMO......
Check at your local airport for certified aircraft welding shops that your local aircraft repair shops use....
If hardened, it would probably need to be annealed first, then welded and re-hardened......if you don't know the hardness, a good welding shop could probably check the hardness for you......and inspect the broken part and advise further......
Hope this helps....
