I used to keep spare screws on hand, particularly the front retaining screw on older S&W revolver sights, since they like to loosen up and eventually disappear especially on harder recoiling magnums. Buggered screws are a pet peeve of mind so I still keep some on hand for new acquisitions. A spare hammer nose (firing pin) or two isn't a bad thing to have on hand for older S&Ws. They can and do break on occasion.
These are handy to keep around for tune ups: https://triggershims.com/cylinder_shims.php
Having things on hand like an ejector, hand or bolt might not be a bad idea. Most revolver parts aren't plug and play, but having the parts on hand would be easier if you have to visit a gunsmith. Particularly with older revolvers that spare parts are getting thin on the ground for. They may be unobtanium when you eventually do need them. Back in the day, the best tool kit was the aforementiond gunsmith, but good revolver smiths are getting harder to find.
Extra guns are of course the easiest, but costliest thing to do. I prefer to have at least three of any working gun, but this isn't always feasible.