One trick I used several years ago was this.  The drive belt was between
the pulley at the top of the organ and the crankshaft pulley, with the
electric motor and a reduction shaft with appropriate pulleys mounted on
their own board and this assembly mounted on the top of the case.

What I did was to take two wooden blocks and bolt them one to the top
of the organ and one to the side, each so they projected out back farther
than the belt and end of the crankshaft.  I then salvaged a piece of
1/2" Plexiglas from the local ice hockey rink and cut that to the shape
so it outlined the belt by two or three inches all the way around.  Then
I mounted that onto the two blocks, using wing nuts on the threaded rod
that ran through the two blocks.

By using the wing nuts it was easy to just spin off the two of them and
lift off the Plexiglas to give me room to open the rear doors so to gain
access to the inside of the organ, for changing rolls, or for any other
purpose.  When not using the organ where people can get into it, I just
leave the Plexiglas off.

At the time I did that, I also removed the lower rear panel of the organ
and replaced it with another piece of Plexiglas so that the pump and
equalizer could be seen.  It's amazing how long people will stand there
and just watch the movement of the pump.

If anyone has any questions about this they can contact me directly and
I'll be glad to help.

Hal Davis