The gods must be crazy.

Often, the choice to keep original and or to "modify" is difficult, there are three "seriously" attended but can't comment on the judging as I have not submitted Vintage Lightweight Bike Shows in the USA, original paint and original specifications will prevail, all of the above in perfect shape will prevail more. IF, and a big if, they are a desireable bike in the first place. In the past few years I have seen, new in the box Raleigh Professionals and a NOS Masi Gran Criterium on ebay, If one took both out of the box assembled them, (with a photo documentation of assembly) and showed them both at the same show, the Masi would win, fewer made and more desire. Period.

Now, lets move to the more common stuff, change a stem to make the bike fit better? I would do it, (gets harder if the stem was originally pantographed) Tires... can be a problem, NOS may not be safe, so be reasonable there. Brake blocks are another possible problem, NOS may not be what one wants to use, at least anymore, a case by case decision.