Engine set back is the square of the motor (rear fins) may not extend more than 17" from center of front axle. No part of the engine or transmission may be more than 21" set back, as measures above.

Does the "square" include the stator and clutch cover or is it the square of the engine case itself?

The Square is the cylinder/crankcase on most watercooled engines. The 21" includes the stator/clutch covers.
Randy

Thanks Randy I appreciate it