Just completed installation of a temp gauge on my 2310... got a great deal on a closeout SunPro mechanical gauge, only $15, but then had to order a metric thread adapter (comes with metric-to-US adapters, of course) which also cost $15.... Oh, well- seems to work, and even came with a light.
I don't remember the size of the adapter (16mm?), but the threads in the tractor were the same as the end of the gauge sender- but an adapter is required for the sender to seal properly. Routed the spring-covered cap tube around & coiled up excess just behind radiator.
Tractor sending unit is located in the base of the thermostat gooseneck, and I found I had to drain radiator (much easier than on most cars!) & remove top radiator hose to get at it. It sort of shows up in the photo, taken from left side of tractor; note side of battery on the right of photo.
I had originally thought an electric sender would be nice, but the mechanical one reads all the time, which I've found nice for seeing how warm the engine is before starting it up.
As an aside, an old motorcycle carb trick-- just a touch of silicone grease on the insides of the rubber air intake tubes really facilitates their removal & replacement!