[wterrell]

Pete,
Perishable, peripheral accoutrements for a firearm should be preserved and repaired. Usually components of a perishable accoutrement should not be replaced (I am not speaking of thread). Replacement of component parts of a perishable becomes restoration, not repair.
An item worthy of repair would be complete, only needing repair, unless the item is so extremely rare that, even restored, it is of premium value because of its great rarity.