If it doesn't feel like a mechanical issue - no noises, no knocks, engine just dies as if ignition is disabled, it's most likely because it is. The tractor has a safety switch under your seat and it it likely thinking that you're not in the operator's seat and kills the engine for safety. The switch is under the seat and can be disabled by unplugging it from the wire harness. Disconnect it, see if it makes any difference in engine operation. If it does, check the switch - the contacts may be corroded or some dirt's got in between them and needs to be removed. Just be sure to connect it back when you're done - it's an important safety feature!