My rule of thumb: do nothing that is irreversible.
Paint is just that: paint - a thin coating applied to a surface to change its color.
It can be stripped-off and re-done an infinite number of times.
Same thing with plating, although the substrate may have limited tolerance for repeated trips through the plating chemistry.
Irreversible changes would be things like sawing-off cabriole legs, drilling holes in cabinets to mount non-original motors / tone arms, etc.
Some of these injuries can be repaired by really talented, patient craftsmen, but they shouldn't happen in the first place. Not in this day & age.
If the piece in question is truly rare, even if derelict and / or incomplete, it should probably be kept in an authentic form ( as-is or properly restored ).
George's point about a well-done horn eventually being mistaken for an original piece is real possibility...
I think if someone does this, they should include some sort of mark or "signature" that distinguishes their work from that of an original.
Unless they are fakers bent on selling frauds for profit, in which case, enthusiast books, and sites such as this can hopefully be a repository for good reference information, to prevent someone from being taken-in by a pretty 'never was'...
My two-cents worth...