There is a middle ground - replace missing parts with similarish but not necessarily totally period parts.
That way you preserve the frame/forks, the bits that won't ever be manufactured again and are only original once.
If somebody wants to fit the exactly correct parts at some point in the future I'm happy with that, at least I've rescued a frame in the meantime.
With British bikes there's also the point that a lot were sold as frames BITD because complete bikes had sales tax on them and parts didn't.
- Just tried searching for the dates on that and it's very tricky to find, but it's just before or after WW2 I think.
So the manufacturer would suggest what parts to use but it was down to the buyer what they actually fitted.
Edit:
Yay, many thanks to nkilgariff
https://www.nkilgariff.com/Models.htm.
In the 1940s to 1970s the UK applied a heavy Purchase Tax on new bikes (over 24% in 1949), but this tax did not apply to all components. To reduce the tax, many enthusiasts would buy a frameset, pick their components and do the assembly themselves. So a high end Holdsworth frameset could have any mix of components. However, a Holdsworth frame stockist in UK would probably have offered the Holdsworthy range of components too, so a certain kit similarity could occur. Many US bikes were bought as framesets and built-up.