MOSTlikely the seat switch is unplugged, bad or needs adjustment.

seat switch OK won`t start if off

***"seat switch OK won`t start if off"***
What exactly does that statement mean?
By "off", do you mean "unplugged or disconnected"?
The seat switch connector has a shorting shunt built into it........if the seat switch connector is physically disconnected, detached, or unplugged from the seat switch.........the shorting connector shunt is invoked, and that will ground the ignition....preventing the engine from running.
Similarly, if the seat switch is defective (and shorted) or the activating means is out of adjustment, the seat switch will be invoked.
However, the seat switch and its ability to kill the engine ignition is ultimately inhibited by the clutch switch and the PTO switch, meaning that by depressing the clutch pedal, or leaving the PTO OFF, the engine is permitted to run, even if the seat switch is invoked.
By virtue of the fact that the engine dies when you let up on the clutch pedal or engage the PTO verifies that those 2 switches are functioning correctly. There is something wrong in the seat switch circuit that is duplicating a "VACANT SEAT CONDITION", so the stage is set (as to the seat switch) to kill the engine if either the clutch switch or the PTO are called on, and you already proved those switches are good.
Verdict.........seat switch or adjustment of the operating means, concurrent with Bill's first assessment.
Post the full, technical model number of the tractor from the ID decal usually found under the seat in the cavity. This will enable us to view the schematic for the tractor to see if any operator presence relays are incorporated on this machine.

THANKS FOR THE GREAT HELP

You are very welcome. And thank you for providing the information needed to help you.

Apparently we provide help in spite of the posters best efforts to NOT provide information or feedback on what fixed the problem.

***"THANKS FOR THE GREAT HELP"***
Actually, I inferred that to be a sarcastic "tone of voice", implying that instead of providing help.....we just asked a bunch of dumb questions.
For crying out loud, it's a free forum.....nobody gets charged a dime to come here and pose a question, but some posters seem to have an attitude that we aren't trying to help if we can't flip them an answer right from the start.
Forgive me if I took it the wrong way but it just felt flippant to me.

I agree with you mownie.
(I noted the sarcasm)

We ask a few questions to help guide us and the OP refuses to answer any of them.
IF the problem was fixed, the OP could at least have the courtesy to say what fixed the problem so that others can learn.

Bill Mownie: I think you satisfied the Op as to what direction to look . Although obvious to the educated he may be a novice within such repair / testing especially the electrical components . I agree that it is respectful to reply back with a form of feedback as to what he finally found.
You can take a Horse to Water , but may have to drown it to get it to Drink !