1994 200 Yamaha Overheating

[EastEnder]

My dad has a set of 200 Yamahas on his Regulator. They are '94 motors. One of them is starting to overheat at crusing speed. The water pump is working or at least the tell tale. Could it be a bad thermostat? Where / what should we look at first. Both are low hour motors.

Thanks, Tom