Don't bother with a temp sender. Yanmar designed these things right in the first place. Replace the sender if you are concerned about it.
It seems every new owner - me included, 14 years ago - starts off thinking the tractor needs a temperature gauge. It doesn't. The warning light is sufficient to tell you it is about to boil over, which should never happen if the cooling system is up to spec and you don't let mowing chaff build up in the radiator fins. Any operating temperature short of boilover is within Yanmar's design specs.
These were designed to run for days flat out without overheating, if the pto is powering a ditch pump, for example, and nothing we do putt-putting around is going to stress it like that. Yanmar got it right, there's nothing to improve.