I dont think there is any science to this. In my experience in buying vintage instruments, if its 100% original, then its exactly that. So if there are changed parts, then, if known, they should be declared. Its not always easy to tell with screws etc though, but it depends what you are looking for, or what instrument you are looking at. Prices will reflect originality, and some items affect more than others. Its certainly possible to find original, unmolested 50+ year old instruments with perfectly working components, and these are desirable.
The exception, from my experience, is original factory strings. I've seen a few instruments claiming to have original factory strings, but personally have not seen this add any value, as its almost impossible to prove, and, its highly unlikely too, especially on a worn instrument. Someone else used the 'car tyre' analogy, which i would tend to agree with, although probably easier to prove an original tyre, than original string on something 50+ years old.
If its not a highly collectible instrument, then a changed pot, or bridge, or strap button is unlikely to affect value too much.