Seems like you should be able to test whether the thermostat is working. Fill a pan with enough water to submerge thermostat completely, put thermostat in, heat slowly with a thermometer in the water and watch to see at what temperatures the thermostat begins to open and also when allowing it to cool at what temperature it begins to close (assuming it is functioning). I've never done this with an NSR thermostat but have with other bikes. Good luck with it.
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-Jim
'93 MC21