This is not a "pipe deam". I have converted many of these "BP 1873" Uberti's to centerfire. As long as the pressure isn't high they work fine with just grinding/filing the firing pin to make it more "centered". Doing this leaves a gap around the firing pin and higher pressures will cause the primer to flow back into this hole.
Installing a 45 (acp or Colt) cylinder is the same protocol as fitting one to any SAA. This requires setting the cylinder gap by fitting the bushing. By the way a "44" black powder revolver has a .451 to .454 bore. Of course caliber changes are also possible by changing and fitting the appropriate barrel and cylinder.
It is legal by Federal law to make your own firearm (state laws vary). You cannot make a firearm if you do so with the intent of selling it, this is manufacturing. But YOU CAN sell a firearm you made by federal law. There is no need to perpetuate the myth that if you make a firearm "you can never sell it".