Tony,  stay with the original Wurlitzer countershaft.  Mount the
motor and countershaft on a board that will sit on the garage floor.
Make up a longer drive belt to the great wheel.

This way the original equipment and speed ratio stays correct, there
is no damage to organ case and all can be returned to original by
simply placing the board with the mounted motor and countershaft back
on top of the organ, driving the great wheel with a different size
belt.

A simple, fast and non-intrusive solution!

Ken Vinen
Aylmer, Ontario, Canada