When the switch is in the on position is there 12v power to the ignition coil on a four cylinder gas tractor?

In the old days the coil was fed by a resistor in the on position. While cranking, the resistor was bypassed and the coil got full voltage.

When the switch is in the on position is there 12v power to the ignition coil on a four cylinder gas tractor?

Yes, there should be power at the coil positive terminal.
The "Negative" terminal is actually the ignition trigger. When it gets a trigger pulse, the coil field collapses and the coil fires.
One terminal will light a test light with the ignition in the ON position and the other should flash a test light during cranking.
I hope this helps.