Plywood is stronger than solid wood.  The latter is more prone to
cracking, and it warps unless it's perfectly quarter-sawn (with the
grain lines perpendicular to the long surface).

Plywood was used for large pumping bellows in most player pianos for
the same reason that almost all piano cabinets were crossbanded and
veneered: to reduce warping and cracking.  The plywood in most player
piano bellows had several relatively thick layers of poplar, which
holds screws better and looks better than the common fir plywood that
was ubiquitous for wall and roof sheathing in tract homes until OSB
came along.

If you make a new pump out of solid wood, it will be vulnerable
to developing cracks when the moisture content changes.

Art Reblitz
www.reblitzrestorations.com