I'm not familiar with the Isuzu cutoff switch, just going by memory of my old Land Rover diesel - Could you remove the fuel cut off switch and try? You'd probably have to stop the motor by putting it in gear and letting the clutch out with the brakes on. Possible test without buying a new switch.

If it's an electrical switch like I'm familiar with, have you verified it's getting voltage?

Again, I'm not familiar with that motor, just throwing ideas out hoping they might be helpful.

Paul