An appropriate right angle gearbox can be had from old reliable
W. W. Grainger, which has stores just about everywhere.  Alternatively,
one can be had from McMaster-Carr, which is a bit easier to deal with
and has lower prices.  Both offer online ordering.

It might just be easier to reinforce the case on the side and get a
motor with a mounted gearbox (a "gearmotor").

This isn't going to be exactly cheap.  In the horsepower range needed,
the gear boxes run something in the range of $400 to $600.  Perhaps you
could mount the existing countershaft assembly on the side of the
organ, making a new mount for the motor that uses its weight to keep
the belt tightened.  That's often done.

Dave Vincent