I think it comes down to what you want the bike for - to ride, or to look at? If the parts are original and they work, although maybe they aren't cosmetically perfect, then leave them on and use the bike like that. If they are original but knackered (or don't work for you, maybe the gearing is too high or something), and you want to use the bike, replace them. If it's all rare, and you only want to use it everyso often, that's fine, but if the parts aren't functional, you're looking at a tubular piece of ornament - which is fine if you want that, but a bike deserves the chance to get out, I think. Just as vintage cars should be driven, or vintage aircraft flown, even it it means making the odd compromise. Unless it's one of the last few of it's kind, keep it out of a museum.
I'm building up my Galaxy (not sure of date, maybe 80's) on the basis of what I can afford balanced against what suits me - but it lost any pretence of originality long ago when we resprayed it.