The best glue for attaching ivory is hot hide glue.  Since the
ivory keytops are translucent, you'll need to mix in titanium oxide
(sho-white) with the glue crystals at a ratio of 1-to-5.  If you have
a gram scale it's easy: mix dry, add water.

I have a few organ ivories kicking around.  Let me know the dimensions
to see if they match.

Regards,

Jon Page, piano technician
Harwich Port, Cape Cod, Mass.