Nothing is perfect. Antique and vintage clocks often suffer the ravages of time and neglect and a neglected old clock is far more difficult to bring it back to life if it is missing crucial parts or repairs have been crude and haphazard. Improper restoration can significantly decrease or destroy the value of some pieces. However, the repair of a botched restoration might be the right choice in certain circumstances.
A complete clock is always the first thing I am looking for when adding new clocks to my modest collection. Dealbreakers are usually broken and irreplaceable tablets, replacement movements, missing crowns or toppers, and unreadable (ruined) dials. A clock that is complete and not altered is always preferable to a clock that someone has “worked on”.

I may consider clocks that have a refinished case if the standard of refinishing is high. I am almost never opposed to cleaning a clock of grime and dirt and applying a coat of traditional shellac (if needed) if that was the original finish or remediating minor veneer issues. A missing finial can always be replaced with a suitable alternative, clock hands can be replaced and some clock movement parts that have worn out can be purchased from a variety of sources.
Does the total restoration of a Junghans Crispi wall clock (above) with many new parts some of which are not faithfully reproduced have value? Perhaps not, but the buyer might be pleased with the end result.
One might argue that any change can devalue a clock but most collectors I know are content with making subtle alterations to the clocks they work on if it means improving the look of the clock, putting it in running order, and presenting it as more desirable for resale.
I recently read a very interesting article (no link due to a paywall) that posited that in select cases reproduction parts may not devalue an antique clock and in fact, might enhance its value if the parts were made exactly the same with the original machinery.

The clock mentioned in the article is an Ithaca double-dial calendar clock that sold at auction for close to $20,000 despite eleven “new” reproduction parts. The photo above shows an example of another clock made by the Itaca Co. in the 1860s.
I was intrigued by the repair and the article went on to mention the work of Joel Warren. Mr. Warren operates a business called Ithaca Calendar Clocks Co. The company specializes in making reproduction pieces for Ithaca clocks. Mr. Warren has a past association with the Ithaca company and possesses the original circa 1895 shaping machine, knives, and tooling from the company. He can reproduce arches, circles, and ovals of varying lengths and widths.
Back to the question: do reproduction parts devalue a clock? The answer as you would expect is yes, no or it depends. Two experts looking at the same clock might agree that the quality of the restoration might enhance a clock’s value but disagree as to the extent of restoration particularly if the restoration goes too far when most or all of its original components are replaced. Is it then the same object?
Case in Point
In what circumstances is the repair of a botched restoration the right choice? A bad repair or restoration of the case might be irreversible but structural repairs should be addressed.
I won a Scottish tall-case clock at auction 3 years ago. It was a relatively inexpensive acquisition but the clock came with a number of issues. I won’t go into the repair of the movement but a lot of work went into rectifying the strike side.
There were issues with the case but the main problem was a detached backboard. The broken backboard (held on by crudely applied modern finishing nails) certainly made it easier to transport the clock in my station wagon because it fell off when I attempted to move the clock. I knew that it had to be repaired otherwise the bonnet would not be stable.

I first had to strengthen the side mounts with high-strength hide glue and slotted screws salvaged for an old clock case. It was not enough to screw the backboard which was in two sections into the side rails so, I fashioned a cross brace of salvaged pieces from an old 140-year-old ogee clock.

The structural repair was necessary because a previously unstable backboard is now much stronger and able to support the bonnet.
Other than cleaning the case and dial and applying two coats of traditional shellac to the wood surfaces the front of the case is as found.

Nothing is perfect but some flaws can be addressed with minimal intervention without taking too much away from an antique clock and might even enhance its value.