Howdy Again
The original question was:
"Does anyone know if it is possible on modern reproduction SSA 1873 models to swap a Bisley style hammer for the standard hammer?"
My answer concluded that no, they will not swap.
Spend enough on custom gunsmithing, and anything can be made to fit.
Jim Watson mentioned Elmer Keith's No. 5 revolver. One of the best known examples of customizing a standard SAA.
Elmer's No. 5 went up for auction back in 2015. Ian McCollum did one of his terrific videos about the No. 5 at the time.
I have taken the liberty of taking a screen shot from Ian's Video and posted it here. I hope Ian will not mind.
Elmer's No. 5 revolver had a hammer that resembled a Bisley hammer, but it was actually welded up to make it look that way. You can see the hump on the hammer extends back like a Bisley hammer, and the backstrap of the revolver is a modified Bisley backstrap. However I saw a photo of the gun with the grips off, and the hammer does not have the T shaped stirrup of a true Bisley hammer. Instead it has the little roller like a standard SAA hammer. The hammer spring is not a Bisley hammer spring, with the two hooks that pop over the stirrup. The hammer spring looks to be custom made, in a V configuration. The roller on the hammer rides in a shallow groove in the hammer spring, much like a standard SAA hammer. So somewhere on that hammer there was probably a weld line at one point, where a standard hammer and a Bisley hammer were married together.
Here is a link to Ian's excellent video about Elmer's No. 5:
https://www.youtube.com/watch?v=r-ef3z9Sv3s