My reaction is to tell you, screw it and just ride that puppy for a while. Overheating is rare for these scooters, but the sender unit for the gauge can be faulty. It's the same as senders for Honda cars of that era, so take it to a car parts store and get a replacement. The sender is on the thermostat housing and has nothing to do with the fan on the radiator, which can run without you knowing it because it's not too loud. That has it's own sender and, yes, you can wire around it and use a manual switch. Or, just set it up to run whenever the key is on.
Anyway, I still say, ride it. Report back after 3 tanks of fuel. You should get between 60 and 70 mpg unless you really hammer it or ride only with a wide open throttle.