Then .. You will need a seat cushion with safety switch setup as NO.. and when the person is setting in it.. it is 'closed'. Make this part of the fuel cutoff solenoid circuit. If you can't find a NO cushion.. but can only find the NC cushions like the lawnmowers use, then you just have to get creative. Install a relay inline with the cutoff solenoid.. use one that has NC contacts... now for powering the relay, attach power from the switched sideof the ignition switch to one side of the coil.. for the ground of the coil, run a wire to one spade on the NC cushion, and the other spade of the NC cushion to ground. Thus, when the ignition is on, and the driver is seated, the relay is NON powered, and the contacts are closed, letting the fuel solenoid function.. if the driver gets up, the seat cushion goes returns to NC status.. completing the circuit for the relay.. its contacts open, and the fuel solenoid fall out.. tractor stops.
All that said.. I'm not a fan of booby traps like that. i can think of situations where this might harm the tractor.. like making it shut off after hi rpm and the turbo hadn't cooled yet, if the driver got up to fast..
Soundguy