Every part _can_ be made, and eventually, likely, every part will be made in order to keep these things operational, but enthusiasts find these machines more authentic if they are made from authentic parts.

If one starts building new parts from scratch there may come a moment where people say something has become a replica, more so if one also uses new technology (philosophers discuss this as https://en.m.wikipedia.org/wiki/Ship_of_Theseus).

The weird thing, though, is that the road along which one gets to an object is more important than the end result in determining authenticity. Replace all parts of a spitfire until just one piece remains original, and you have an original spitfire. Build a replica, buy that one original part, and include it, and you still have a replica.

A nice example is HMS Victory, a wooden warship that saw battle at Trafalgar. It is kept afloat by replacing its parts. By now around 20% of the original wood remains (http://edition.cnn.com/2011/12/05/world/europe/hms-victory/). Yet, we think of it as the original ship.

However, it is not a given that replacing all parts makes a new object. The Ise grand shrine in Japan is over 2000 years old, but gets rebuilt with new materials every 20 years (https://en.m.wikipedia.org/wiki/Ise_Grand_Shrine)