The core concept of literally any collector is to have a vintage watch in the condition as close to brand new as possible keeping at the same time the absolute majority of parts original, as it came from the factory. That is why redone dials (even those which are masterpieces of refinishing when the differences with original untouched dial are uneven) decrease the value of the complete watch dramatically. Replaced bezels, hands, crowns, movement parts are less dramatic but still very significant as it still affects a lot the integrity of the watch.
In the row of key important factors and criterion, timekeeping (but not functionality and condition of the movement) stays among the last, as for example the presence of an original buckle. For instance, all major auction houses sell watches unserviced and often unmaintained, and the only thing they guarantee even while selling million EUR worth Patek Phillipe - is authenticity and that the movement works. You can always read the line "overhaul recommended at buyer's expense". The absolute majority of vintage watch dealers sell watches without any service, and often with only 1-month warranty not covering timekeeping.
Therefore, dealing with vintage watches, we are focused on "soft" restoration keeping as many original parts as possible even when we talk about the movement. There, apparently, parts which can hardly survive after 40 years - like mainspring or hairspring of the balance wheel, therefore these may require replacement, as well as broken parts. However, it very important to understand that there is no sense in replacing the part which is worn after 40 years, but not critically. As soon as watch movement is extremely precise, we are talking microns, rather than millimeters. In other words, a replacement must be clearly needed.
In fact, it is hardly possible to source for example brand new (NOS) escapement or a balance wheel (which still needs to be adjusted) for IWC cal.61. Even manufacturers often have no original parts and they use freshly made replacement parts. No vintage, no integrity. With some watches you simply can't do this - let's take original IWC Portuguese or and Patek Philippe Ref.533 from the 40s. Brand new unworn parts simply don't exist. Only used or newly made can be used for repairs (like Patek Philippe is doing now), so you often have to face a choice - you gain in precision or you preserve the value.Therefore, unfortunately, we are not able to guarantee timekeeping in a long run and have to add a special limitation to the warranty terms. We guarantee that every watch is fully functional, that all functions are working correctly, that it keeps working for at least 30 hours after being fully wound. However, we do not guarantee any better accuracy of -3 minutes / +3 minutes a day. Even though we strive to achieve the best possible performance when we service our timepieces.