This being a ride-on mower, it has half a thousand delightful safety devices to keep us from putting our face in the blades then pressing the starter. Some of these devices work by triggering the magneto's kill wire: the wire that we ground to switch off the engine. So, there probably isn't anything wrong with your magneto, you probably have a displaced wire or a broken microswitch on one of the safety devices. You need to check the driver's seat, clutch, cutter engagement, and whatever other microswitches it has. My preference is to trace it all on the wiring diagram, but lots of serious techos just go to each microswitch and check its status. You can start by checking whether this is the actual problem by just establishing whether the kill switch wire from the magneto is grounded. If it is, try temporarily un-grounding it by disconnecting it. Of course, be very careful: without that device operating there is nothing to keep you from having your foot or fingers in the cutters when you crank it - and no way to stop the engine. Once you establish that it will run with the kill switch disabled, you have to trace the circuit, checking which switch is grounding it. You didn't try to start it without sitting on the seat, did you?
