As a real novice, I purchased a 1924 Model T touring that was 99% original. It did have new safety glass, and some new wood. The mechanic I use wants to put on new parts whenever something goes wrong. For example, the spring that holds the timer in place broke, and despite my request to repair it, he ordered a new modern time without the rollers. He believes dependability is more important than originality. Does the new parts decrease the value of the car? He is saving the removed parts for me at my request.
No. New parts increase the value of the car, as a more drivable, dependable car tends to be worth more.
All moving parts will ware out at some point. Roller timers, tires, etc were replaced back in 1924 just like you replace them now.
Most of the newer timers look very similar to the original, and most people looking at your car would not know the difference unless you point it out. If it runs better, it is a good replacement. Just keep the old parts and if you sell the car someday, they can be sold with it, so that if someone wants to make it "original" it will be easy to do so.
The timer (commutator shell and roller) were never part of Ford new car warranty.
Replacing such non-warranty parts is fine, won't hurt and will improve value.
Timers rarely last 5,000 to 10,000 miles, either the roller is worn or the shell is worn.
To check your old timer to be sure it was a Ford original, look under the timer rod attachment, and you will see a script "FORD". If not, the timer was replaced long ago anyway
Sometimes stamped FORD upside down
Smart move Annette, requesting that he hold the removed parts. Ask him to give you the old parts and post pictures of them on the forum, I'm sure some here can give you an assessment of their condition. If the parts are proclaimed to be good, you can bag them and save them for emergency spares.
Just to be clear. Replacing things like timers or tires with proper new replacements is one thing. They help the car, and generally add (slightly) to its value. Then again, some components can be replaced by totally modern parts that do not look or behave like the originals. They may or may not work better. And they can and may make the car less desirable to people that know and want a real model T. That, in turn, can reduce the value.
Many times, people will spend a great deal of money putting all kinds of brass where it doesn't belong, painting things wrong colors, doing upholstery in ways that were never done back in the days, and adding all kinds of things that simply are not correct to a real model T. When they go to sell it, they are usually surprised to find that nobody is willing to pay even half what they spent on it. The car was what THEY wanted, it was THEIR vision. Lots of people are interested in model Ts that are near correct (they don't have to be perfect). All the individuals that want a car that fits THEIR vision has THEIR OWN vision, they don't want someone else's vision.
Keeping a car close to correct while making sure that it is running and driving like it should is the best way to keep a model T.
Drive carefully, and enjoy, W2
There are some really poor quality reproduction timers out there. Some last only a few miles before wearing out. This is a very important part of the car. Without a good timer the magneto might not work, or the car might leave you stranded unexpectedly. Just because a timer is new or expensive does not mean it is going to be good.
It pays to be part of a local club. You will get honest information and save yourself a lot of heartache learning from other's mistakes by listening to them.