You are right, red dragin in stating that a 100 year old loco is not all original. The boiler is one major component that would have been replaced as a matter of course. The boiler must undergo a thorough inspection by a qualified boiler inspector on a periodic basis. He may recommend replacement of stays or firetubes, or welding of specific areas, or if it has deteriorated to a level where repair would not be cost effective, may condemn it. How long a boiler will last is difficult to foretell. It may be as little as 25 years, or even more than 50 years. A lot depends on water quality, water treatment chemicals, and the storage situation. If a boiler is left unused with water in it for months on end, corrosion will make itself felt. For long-time storage the boiler should be emptied and left dry. Boiler plate may be15 mm thick or more, so there is a good factor of safety, but corrosion is a killer.
It is surprising just how much is original, though. Original components on our engine include the mainframe, wheel centres (tyres are replaced when worn to condemning level), axles, valve gear, cylinders, (yes, the cylinders ARE original), sand dome, steam dome casing and more. The transformation carried out to work at Dreamworld included provision of new components - cow-catcher, smoke-stack, cab, headlight, bell and other minor decorative components.