I work for the Tweetsie RR in Boone, NC. I am also a volunteer for the ET&WNC RR. I have had the privlage of being involved in the restoration of 6 locomotives in the past 4 years. I have not seen anything yet that can not be repaired or replaced through machining and casting. The number one thing that puts most steam locomotives out of service is the boiler. The boiler plate can crack, or get thin through pitting. Also a busted frame could put an engine out of service.
As for cylinders, as they get worn you can bore them into round, and or sleeve them. The pistons can also be built up with brass and have larger rings made.
Im 25 years old I for intend to do everything I can to make sure Steam is around for generations to come.