If you own or intend to buy an antique clock, the all-important question arises:
Is the clock worth the restoration that you may be considering?

To answer that, you must bear in mind that the answer is partially subjective; indeed if the clock has a sentimental value (for family reasons or otherwise), then you have the answer already.
If other considerations come into play, as a professional restorer of clocks, I can help you making the decision by providing you with three elements:

1. A complete description of the clock with details of its era, maker and manufacture, history, etc...
2. A detailed study of all its physical qualities and drawbacks; the quality of its manufacture, its originality (do all the components originally belong to it), its completeness (anything missing, maybe?) and its present condition with all its possible problems.
3. A complete quotation for all the essential repairs and restorations, along with optional repairs that may or may not be carried out.

Don't forget that what makes the true value of a clock are the three basic elements:
Age, quality and condition.
For more details on those, see the "Appraise" section.

In our workshop, our main purpose and responsability is to restore clocks so as to preserve its original character and properties.
If parts are to be replaced, the new ones will always be manufactured so as to resemble the old ones as close as possible, in the spirit of their maker.
Repairs are carried out with the principle of reversibility; this means that, wherever possible, the treatment we apply in the form of adjustment, repair, modification or replacement is such that it will be possible to put the clock back into its preceding state.

When a part is so worn or damaged that it must be replaced, the overall appearance of the clock will not be spoiled by an obvious difference in the character or craftmanship of the replacement. But we will not necessarily match exactly the original material or its colour, nor will we attempt to give an aged appearance as this would constitute faking.

However we also consider that all repairs carried out previously by other craftsmen should be preserved, as they constitute the history of the object, all the more so if these repairs are made in an unusual, interesting or inventive fashion. (For an illustration of some of these, refer to the "Oddities" section.)
Previous repairs will only be undone if they were obviously badly carried out or defaced the clock.

Ultimately, most of the restoring work is a matter of personal judgment, in which the restorer (me) will always keep an open mind and communicate with the owner (you) as much as possible.
There are no absolute rules, just endeavours.