Ruger will replace parts that were original to the gun. If the gun is marked .45 Colt, and you send it in w/o the spare cylinder,, then they will most likely replace the cylinder. What they won't do is add a cylinder if the gun wasn't originally shipped as a convertible. But the Colt cylinder is the primary one so yes,, they could build another one.
BUT,,,,,,,,,,,,,,, the fact it's an Old Model,, as noted,, they will do the safety upgrade,, even if asked not to. It's automatic. And here's the fun part.
There is a difference in an OM cylinder vs a NM cylinder. Ruger may NOT have the parts to get you a correct cylinder. Yes,, they can make one,, but it will cost.
If it were my OM .45 I'd prefer to hunt the spare parts places to find an OM cylinder that matches the OAL of your existing .45 acp cylinder,, or maybe a few thousands longer. If it's the same length,, it'll drop in & be fine. If it's a little long,, you can gently hone off the front cylinder boss to fit. Spare cylinders are out there,, and that would be the place to go.
However,, if you choose to send it to Ruger,, I'd remove the hammer, the trigger & the base pin to make sure you retain the original parts.
Expect to pay around $200 for a spare cylinder.