I am currently restoring a Poppers Konzertist piano and can answer
some questions raised.

The box pump uses a round section leather belt.  If you cannot find a
suitable supply of this you can use a modern replacement round section
plastic belt.  This is readily available from your local bearings and
belt supplier.  It is available in various diameters from 3 mm to 12 mm
and is hollow so is connected together by a small barbed metal insert.

It is reasonably flexible so you don't have to bother with moving the
motor to tension the belt, you just cut a bit more out of the belt and
refasten with the insert.  You should be able to find a local supplier
that sells V belts, pulleys and bearings, and they should be able to
help.

The Poppers Konzertist will play the wide Poppers standard orchestrion
roll.  This roll will operate the sustain and soft pedal, mandolin and
accent holes in the roll, plus the play and rewind.  This roll is also
fully orchestrated to play the percussion and pipe ranks on Poppers
orchestrions and the Happy Jazz Band, but when played on the straight
piano these extras are ignored.  You can therefore purchase these roll
for your piano.

Original Poppers rolls can be obtained from reputable dealers in
continental Europe but you can also but new rolls from Thomas Jansen
www.maesto.com who will cut to order.  His catalogue has 194 Poppers
titles and these are all large rolls with either two or three short
tunes or a complete overture or orchestral arrangement on each roll.
They are reasonably priced at 40 euros each, which is about the price
you would have to pay for original rolls, and I can personally vouch
for the quality having just received a batch for my own instrument.

Because the Poppers pianos and orchestrions of all sizes play a common
roll, this piano is an excellent start for anyone wanting to make a
quality 'build-up' orchestrion.

There are, actually, slight differences in the scales of different
styles of Poppers orchestrions so the commonality mentioned above is
not exact, but anyone capable of making a build-up will be able to
deal with these differences.

Best wishes,
Nicholas Simons, England