Hi,  Replacing keytops is indeed easy as Mr. Goodman states in his
posting, however there is no excuse for using white glue in doing so!

My piano tech gave me some pieces of hot glue that were specially
treated for gluing ivory from the 1900s.  They are white in color.
Failing that, a little white titanium oxide powder goes a long way
and you will be able to glue them with hot glue easily.

Additionally, there is a clamp available at Schaff piano supplies that
clamps the ivories down flat and pushes them back at the same time,
giving a perfect seam.

If you are gluing the ivory keytops back in place, then this should
be sufficient.  If you need to replace missing pieces, it will take
a bit of searching but you can find pieces that match in color, shape,
quality and shape (the A, B, C, etc., note keytops are all different).
I recently had to replace 14 pieces of ivory on a Sohmer Welte upright
and I bought 30 old heads and had enough to match "good enough".
A gentle rub with some steel wool will whiten the ivory, followed with
a buffing with polishing compound and you can have pearly whites.

If you have to remove one, a gentle iron is not recommended as I found
it tended to curl the ivory (or it could have been that that piece was
naturally wanting to curl and I just released it from its glue hold).

Eliyahu Shahar