prolly a cuppla safety interlocks involved with that start circuit, but would start with simplest stuff first:
make sure all elect connections are clean to bright metal and tight.
Load test batt. Replace if low
Arc across large terms on start solenoid - if engine cranks, replace solenoid.
next find.chk safety interlock relays/switches and test, including ignition switch(intermittant crank makes this a prime suspect, IMHO)

OK, when you say "CRANK", do you mean the engine spins over but will not start?
Or do you mean it intermittently does not spin the engine when you turn the key?
Many folks want to use the word crank to mean the same thing as "start" even though cranking is the process of spinning the engine to get it running.

"Many folks want to use the word crank to mean the same thing as "start" even though cranking is the process of spinning the engine to get it running."

Ditto,

Walt Conner

Doesn't crank means, turn the key and nothing happens other than the carb solenoid clicks and the hour meter makes it's particular sound.

First off, you will need a test light or DC voltmeter....and a trusted helper to sit in seat and operate key switch would be a plus.
Begin at the starter solenoid. There should be a small Green wire on it. Test for voltage at this small Green wire when the key is turned to start. If you DO GET voltage but the starter does not activate, you need to replace the solenoid. Solenoid might not be available except as a starter assembly.
If you DO NOT GET voltage on this Green wire when the key is turned, there is a fault somewhere in the circuit between the key switch and the Green wire terminal of the solenoid.............and finding that fault may require testing a number of components and connections.
The simpler tests include things like jiggling your travel control sticks back and forth across the neutral position to see if your "intermittent" cranking is due to one of the neutral start switches being defective, or having a loose connector plug on the switch. Hold the key in START position while you rock the travel sticks back and forth across neutral to see if you can ever get the starter to activate. Do the sticks one at a time. Each stick has its own neutral switch.
If you get no response doing that, you will need to physically check the connector plugs on each neutral switch to see if there is corrosion on the wire ends or if the connector is loose on the switch.
There is also a starter control relay involved in the process. The Green wire on the starter solenoid is powered by this relay so you need to locate that relay and check it for corrosion or loose connector plug. This relay can be identified by its have wire colors Red, Black, Green and a Blue with red stripe in its connector plug.
Examine this relay closely for any signs of green corrosion at the base of the metal connector blades where they enter the relay body.
If none of these suggestions help out, post back and I will have another go at the Scag wiring diagram.