Have you checked all the safety switches? Seat, brake, PTO. Do you have power to the solenoid when you turn the key to the start position? Have you tried jumping the two large terminals on the solenoid? Put the unit in neutral, parking brake on, PTO not engaged while doing all electrical tests. I think it is either a safety switch, bad ground, or soleoid. Also, if you have a battery charger, try jumping the starter to make sure it is good.