There is no answer, and yet there are many answers. A first question that comes to mind is do we ever have an unrepaired or unrestored gun....150 plus years old and having served it useful life. Few, if any come with real certification.....only Man made opinions. A wise old collector once told me that anything "man" has once made, he can even more easily remake today. It seems to me that each individual collector, perhaps in consultation with others, must make a decision on each item. There would be purest , then variations all the way to I am just glad to have an attractive "original" for my collection and visual enjoyment and study. Value again is in the eye of the beholder , owner and buyer. The ability of the restorer and quality of the restoration to approximate the whole original becomes a major factor...to be able to replace with period or reproduction parts, another factor. The rarity of the item, maker, etc. is an issue to be considered. Access...many of us are fortunate to frequently have access to potential additions to our collections and thus can wait for a "more original." Then there is the issue of repair or wear/abuse while in use; clearly this is the history of the piece...should it remain or be put back to the original.
Perhaps the most important issue for many of us is that the documentation of what has been done somehow follow the gun.
Hope I have just begun to add to the controversy. The discussion is a necessary one and all should contribute so we can offer knowledge/opinion to each other and to those who are new and interested in this American art form.
I would suggest that individuals contributing opinions pro or con could very constructively guide their comments in relationship to each of the typical aspects of restoration, i.e, locks and conversion, reconversion: wood repair, barrel lengthening, inlays, finish, restocks etc , etc. and how each can be identified as well . Perhaps even a thread about each could be started. It seems to me that opinions would vary amongst each of the elements as well. It is very important that all the experts and collectors, particularly those of us with age, should comment here to ensure their opinion is recorded for ever.