Sometimes you can remove a reservoir and that will create enough
space for the suction box.  Player pianos work fine with one reservoir.
I have seen external suction boxes and suction boxes mounted above the
stack.

I have several salvaged modern Aeolian suction boxes lying around
that I can sell for $30 plus shipping.  They measure 7" x 7" x 8" high.
These suction boxes are not as powerful as those sold by Player Piano
Co. and aren't as good otherwise.  However, these Aeolian boxes should
be more than plenty powerful because your player is a new restoration.

You can also make your own suction box.  I recently saw an Oreck XL
vacuum cleaner inside a player piano.  It was loud, but not that
audible when the music came on.  The customer seemed to like it and
the guy who did it (not me) and the only other option in his case was
an external suction box.  What the heck, it does the job for him and
he likes it better than any other alternative.

Bill Maguire