There aren't very many restoration threads on this forum for a couple of reasons. Number one, few if any of the professional restorers are members of this forum. Number two, the professional restorers I know like to keep their knowledge and tricks of the trade close to their vests.
There are some collectors who prefer to keep guns in "as found condition", but a lot of collectors will have varying degrees of restoration work done on their rifles.
The most common type of restoration on a Kentucky rifle is a re-conversion back to flintlock. This practice began in the US back in the middle of the last century when interest in old Kentucky rifles was beginning, and they had a relatively low value. It has almost become an accepted practice by many collectors today. The biggest issue is that not all restorers understand what cock, pan, and frizzen are correct for the period and type of gun. In the 1950s and 1960s, about the only parts available for re-conversion work were Bud Siler's Germanic lock parts, so a lot locks were re-converted using them--even English locks. Some of these locks have had new re-conversions with more correct parts in recent times.
There are stories of some of the early collectors with buckets of percussion hammers from guns that they had reconverted to flintlock.
Other restoration work ranges from fixing cracks in the stock to repairing broken wrists to replacing small sections of missing wood to replacing the whole forestock with new wood.
One of the most dramatic examples of restoration is the only known rifle with Andreas Albrecht's signature on the barrel. It is shown in its un-restored state in Rifles of Colonial America Vol I by George Shumway as gun No. 46 starting on page 198. In its "as found state", it was just a beat up old stock with the barrel. It was missing the forestock from entry pipe forward, missing the entry pipe, and of course, the forward pipes, missing the lock, missing the wood patch box lid, and missing the side plate. The barrel, trigger, trigger guard, and butt plate were the only metal parts on it. There were also significant pieces of wood missing around the lock and tang areas. The fully restored rifle is pictured in The Lancaster Longrifle by Patrick Hornberger and John Kolar on pages 10 & 11. The missing and damaged wood has been replaced and repaired. The missing metal parts have been replaced including the lock, ramrod pipes, side plate, and screws. The new wood has been "antiqued" and some of the original wood has been refinished with an "antiqued" look. This is not the only antique rifle that has had this much work done to it.
There is plenty of amateur restoration work done, too. Some of it's ok, and some is dismal.
I have a friend that owns a Jaeger with a very unfortunate past. A well respected collector owned it at one time. Its forestock was evidently broken and damaged at that time. After his passing, it changed hands and came into the possession of an individual who decided to take it apart and sell the pieces on eBay. The metal mounts went to one person and the broken stock and barrel to another. The metal mounts found their way to The Rifle Shoppe where molds were made of them. An eventual owner of the stock and barrel decided to have the rifle restored and ordered a set of castings from The Rifle Shoppe. The restorer decided it would be easier to completely replace the forestock instead of trying to glue the broken pieces back together and replace missing sections. The original forestock wood was discarded. The rifle has been restored, but the only original parts are the butt stock and forearm, the barrel, and I think maybe, the lock. This type of treatment may be all too common.