I think the hammer will come off if you mill the surface weld and perhaps use an end mill to bore the hammer screw hole a little. Then clean up the shaft, drill and tap a new screw hole, make a new hammer screw and slightly refinish all the metal as needed. Great to turn a hundred dollar repair into a full days work with man and machine.
The grade four needs other work. Barrels are pitted but maybe not hopeless. They will need to be honed to clean up. Wood has two issues, one minor and one fairly major. If farmed out the hammer repair and wood repairs combined might be more than the gun cost. But I don’t see very many grade fours and saving this one might be worth it.