It could always be the starter. Put your voltmeter on the starter cable and see if the tractor so much as asks the starter to start. Brushes in the starter eventually wear out. If you have power applied to the starter ask someone to hold the key (or button) while you tap the starter with a rock, hammer, screwdriver handle, whatever. When the brushes go bad there is usually one or two more starts remaining that a bit of vibration can enable. This also helps a sticky starter solenoid.
If nothing at the starter then work your way back from there toward the ignition switch. Something between the starter contact on the switch (or your separate starter button?) and the starter is preventing your starter from turning.