Owner's manual doesn't have any wiring schematics or theory of operation, basically tells you how to do basic maintenance and how to operate the tractor.
You'll need a work shop manual, or referred to as a WSM.
You'll have to verify if the solenoid gets +12v when key is turned off. If it doesn't, you'll have to track it down to possibly a fuse, or a key stop timer, etc. If it's getting power and not pulling the lever to the off position, chances are good that the shutoff solenoid has failed.