How to drive the compressor.

I am considering several options for driving the compressor. One way is off of the output shaft and the other is off of the crankshaft end.

Output Shaft
The output shaft is in a location much better suited to driving the compressor. The output shaft only really spins when the engine revs up above the clutch engagement point. Coupling the compressor to it would mean the compressor wont spin at idle, starving the engine of air. The output shaft would be better suited to driving a loading device for engine testing.

Crankshaft End
The crankshaft end is not very accessible and would require a complicated custom coupling system that would mean a rotating belt or chain very close to the driver's seat. It may require significant modification to the cart to work.

The best solution may be an idle air bypass that works in tandem with the clutch on the compressor. This would allow air at atmospheric pressure to be sucked in to the engine at idle, and then the compressor to be used above a certain engine speed.

Possible Solution
A relay could be used to oprate a solenoid valve, bypassing the compressor at idle. At a certain point the relay changes position opening the contacts for the solenoid and closing the contacts to engage the compressor clutch above a certain engine rpm and/or load when the engine centrifugal clutch engages. This would allow the compressor to be driven off of the output shaft. A pulley could be added to the output shaft or drive shaft and a V-belt used to drive the compressor.