I've never seen nor heard of a conversion cylinder for C&B revolvers chambered for the .45 ACP. Not only does the .45 ACP cartridge operate at significantly higher pressures than the .45 Colt or .45 S&W "Schofield" (18,000 CUP vs about 14,000), but the jacketed bullets found in nearly all factory loads would be very hard on the bore. ASAIK, there are no "Cowboy" loads made in .45 ACP, as the cartridge isn't legal for CAS competition.
IMO, such a conversion would be dangerous, and I doubt that any reputable firm's lawyers or insurance carrier would allow it.
FWIW, all .45 Colt "Cowboy" loads use soft lead bullets and are loaded to very mild pressure levels. Before we get into the old "40 gr. of powder" argument, Mike Venturino and others have pointed out that all original military .45 Colt ammo was loaded with 30 grs. of powder under a 250 gr. bullet, and the .45 S&W that was the only issue ammo for both the Colt and Schofield after 1874 used 28 grs. and a 230 gr. bullet. No C&B revolver except the Colt Walkers/Dragoons could even hold 40 gr. charges under a RB, much less a heavy conical. Even with modern steels, pushing the original design envelope too far is asking for grief.
While "Cowboy" factory loads using smokeless powder might be warranted as being "safe" in conversion cylinders, I'd strictly avoid any .45 Colt ammo not so labeled. Handloads using light-to-moderate charges of BP, Pyrodex or similar volume-for-volume substitutes would be best. About the only smokeless I'd personally use would be "Trailboss", as it would be much more difficult (not inmpossible!) to inadvertently get a dangerous overload.