I'll almost always replace a 2-wire cord set with a proper 3-wires
set, and that'll be what the codes say to do.  However, on a piece
of equipment where the motor is electrically totally isolated from
anything conductive, I might consider a 2-wire cord set for historical
restoration.  In this case, isolated meaning the motor is mounted on
dry wood or rubber, there are no metal linkages to it of any kind,
and the motor is totally enclosed so that a person/animal/stray
object cannot touch the motor casing.

So, unless you're doing a total "down to the last brad, got to be
-exact-" restoration, use a 3-wire cord set and ground the motor casing.
Also, any place there are exposed electrical connections near metal,
such as a metal switch box, that metal should be grounded.  (If there
are light fixtures inside the piano near the strings, I'd ground the
plate, too.)

(This is based on North America electrical practice, and "ground"
 = "earth".)

Carl Zwanzig