A replica cylinder does not neccessarily need any fitting. As for antique value, if Marlin93 wants to fit a replica cylinder then something is probably wrong with the original one, or it´s missing. If something is wrong with it then just keep it for show and get a new cylinder and you wouldn´t have hurt value a bit, if it´s missing then making the gun complete with a new cylinder does NOT hurt value. If he just wants a spare cylinder for shooting, then there´s no harm in that either (if the rest of the gun is in sound shooting condition). I don understand where this idea comes from?
However, not all replica cylinders fit an original revolver. The dimensions often differ, the diameter of the cylinder is often different and thus the chambers don´t line up with the bore. I do not know which replica cylinders fit and which don´t, but an educated guess would be that Uberti makes a better fitting cylinder since they are generally a lot more true to original design and dimensions then for instance Pietta.
Add to that the fact that the originial guns working parts may be worn, and the mechanism timed accordingly to that wear to function properly, then yes, even an original cylinder from another original gun may need fitting. It is usually not a huge problem, and more often than not, it´s no problem at all.
There are new cylinders sold specifically for original C&B revolvers. Check out Dixie Gunworks catalog, look under "general merchandise"-"parts"-"original handguns"-"Remington".
I´ve had several original Remington and Colt revolvers with new cylinders (cheap beater/shooter condition) and some have been perfectly matched to the gun, a few have had problems with the chamber/barrel lineup.
What you definitely SHOULD do before you shoot your original handgun is to have a gunsmith or an experienced blackpowder shooter check it over first. I don´t know what your experience is with blackpowder shooting in general or shooting antique guns in particular, I don´t even know if shooting actually is on your agenda or if you just want to make your gun complete, but if you´re not expericenced then get help.
Anders Olsson