Just replaced mine with one from a 1990 2.0s Honda Prelude. Part number Mahle TX783D in the pic below. I left the little one ball way valve in place as it gets the bike up to temperature quicker and it now regulates as you'd expect, physically dropping from full gauge (below the red) to around 3/4 and even as low as 1/2 when riding on a faster stretch or when the fan is in.
Also replaced my coolant temp sensor with the Intermoto 52470 meant for Honda Accord, Prelude, Rover 200 etc cost me £23 for both from my local factors.

To save having to burp the system, leave out the coolant temp sensor and top up the bike with coolant until it comes out of the sensor pocket (it's the highest point in the system) then block it with your finger before replacing the sensor.