It's most likely that the real issues in the restoration of vintage bikes are the honesty and transparency of the bike's owner. Originality is very hard to come by in a vintage bike and can't even be suitably defined. Even contemporary catalog images of vintage bikes are now found to have been retouched. I have been shown heavily and poorly restored vintage bikes by their original famous makers in their own shops and been told these bikes were original. Collectors here then told me that these great makers don't remember their own original work. Original means pristine. There are very few, if any, pristine vintage bikes around. Fake vintage bikes are always advertised as all original or 100% mint, etc. I think that what many collectors mean by original is based on their own concept of originality predicated on their experiences with the bike in question or with images of it, etc. This is very slippery ground since these concepts of originality are ideas and tend to change with time and place. Even the experts can't agree. Bike parts may be helpful in determining a vintage bike's age but they can be easily changed and makers sometimes sold frame sets or full bikes with a mixture of parts from different periods. Many parts can't be correctly dated either. As far as restoration goes, it's either well done or not. This becomes highly subjective-if it's pleasing to the owner or expert it's called conservation, sympathetic restoration, etc. Someone selling a bike has almost always conserved it. Sooner or later, almost every object created by man needs to be restored. No one can be against restoring part of an object unless the failure to to do so endangers the whole. However, even this a subjective decision and varies depending on the culture, time, place, etc. Furthermore, although sometimes the " don't fix it 'till it's broke" concept is a good one, however, if one waits too long to restore a bike, eventually it can't be restored at all. In sum and in short, the time comes to most all surviving vintage bikes to be restored. All the vintage bike collector can hope it that the restoration is well done and the bike's owner is honest and transparent about it.
-- George
George Hollenberg MD
CT, USA