Replacement of critical parts like hammers always has the chance that professional pistolsmithing might be needed.
One aid to installation is to remove the double action strut on the front of the original hammer and install it on the replacement.
Also note that over the years hammers did change in small details that can cause problems.
As example earlier hammers lack the circular clearance cut on the right lower side to clear the hammer block safety assembly.
DO NOT ever alter the single action notch of the hammer or the sear tip on the trigger.
If these have been altered in any way, the part should be replaced.