A cylinder off of Ebay may not fit. You MAY get lucky, but they are "fitted" for a reason.
I found a shop with some ACP cylinders, out of the 4 or 5 cylinders they had, and the 3 guns I tried them in, 1 cylinder fit 1 gun.
Something else to be aware of, if you send a gun to Ruger for any reason, they WILL remove ANY non-factory part or modified part and replace it with a factory part, and you do not get the custom part back.