Restoration is the term used to denote alterations to an organ to bring its condition back to when it was first built, or at least some significant stage in its history; undoing modifications subsequently undertaken. Simply taking an organ apart and putting it back together with things like leather and screws renewed is merely an overhaul, not restoration.
Similarly, modifying an organ by introducing elements which were not there before and altering basics such as internal layout, come under "rebuild", not restoration.
Below is a growing list of articles under the banner of restoration of mechanical organs.
Emmett's 89-keyless G4 Gavioli
Irvin's 89-key VB Marenghi