Try arcing across the starter solenoid and see if it will crank over that way. If it does, there is a clutch or seat safety switch problem. If it has an electric PTO switch, sit on the seat, turn the key on, then pull and push the knob and listen for the electric clutch to click.
If nothing still works, check all connections under the backside of the dash.
From there you will need a multi-meter and a test light to do some circuit testing to find the issue.