There are little to no engines out there in running condition that have all the original parts and or no modifications from their original design. Granted in #9's case it had to be done all at once, however all early engines had design flaws, and overtime they were worked out and originals modified as needed.
So that is the argument given to the "purists" if you will. Think of all the work these bigger engines require for a rebuild, like the 844, Challenger, and many others. They have modern things like speedometer, air horns, 26L brakes, event recorders, etc because they need to meet certain standards. So a railroad museum or tourist railroad rebuild or restoration project is far more historically correct. The bigger and more IMPORTANT thing to look at here is this : Any steam locomotive preserved or returned to operation, regardless of how it got there, is an important step into preserving our countries history.