Some things would have replaced naturally over time . I fitted a cotter less TA ally double clanger to my 70's Holdsworth Record in the 80's as I found the original steel single crank to be too low geared . To me it was a natural improvement being light , offering different gear ratios and also looking good . In those days the idea of maintaining a bike in original condition wasn't important . I think the tyres may be from the 70's.
I am at the moment working on a late 70's Falcon Professional frame . Trying to find out what model it is and what equipment it would have had is being a pain . It could be that it was bought as a frame only or built to a customer's spec is also possible . The frame came with some Weinmann gold anodised type 500 side pull brakes which might be the originals. I don't know . I may fit them , I haven't decided yet . I might go for some later Campag dual pivot brakes along with chain set and rear mech , it is still to be decided yet but that is the way I am going at the moment .
As I have said people might have started off with a basic bike in the first place and as time and money allowed bought components to upgrade or to replace worn out parts , so to find completely original bikes will be difficult .