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Wondering why you didn’t replace the impeller while you were rebuilding the gearcase? Never mind, that’s in the past. You can test the light circuit by sliding back the rubber sleeve that covers the connection to the tan wire that enters the cylinder head, then ground the wire (key turned on). Bulb should light. But this is not much of a test. It does not test the thermal switch in the head. Also, as anybody knows, a bulb can light today but be burned out tomorrow. Besides, you probably won’t even be looking at it when it comes on. That’s why they went to a horn in later years. Nevertheless when all is said and done, the best solution is a known good water pump and don’t be worrying about it.
There should be a light spray of water anytime it is running. When it reaches a certain temperature, the thermostat opens and water discharge increases. Keep in mind, it is not a simple matter of open or closed. The thermostat constantly monitors the temperature a opens or closes as needed to maintain that temperature.