If the tractor isn't boiling over, it is probably a bad temperature sender. That part is known to fail after a couple of decades, and it usually does so by indicating overheating like you say. I can't think of an easy way to check the sender because it normally doesn't close until well above the boiling point of water. Some of the guys tried things like submerging the sender in hot sugar or oil solutions and measuring the triggering temperature with a candy thermometer. You might try that. Or better yet, just replace the sender with a new one. They aren't expensive and will probably be in stock at most dealers. Or you can do like I did on my YM165D (about the same tractor), and replace the indicator light with a real automotive temperature gauge.
