Both tach and speedometer on K bikes are connected to the computer control box under your saddle, they operate totally on electric sensors. There are features on the K bikes that depend on those sensors to not only display your speed and rpm but to also auto cancel your signals and prevent your motor from exceeding its rpm red line. The speedometer sensor is located in the rear hub/drive shaft assembly and has a magnetic pickup which is prone to collecting any metal shavings floating around in the oil, cleaning that sensor will sometimes make it work again.
So unfortunately the correct answer is: only the original equipment will work, anything else will result in slightly crap results.
btw: Keep lots of fuel in your tank, your fuel pump is huge and dependent on the fuel for cooling, if you run it with very little fuel, the gasoline in the tank can actually reach boiling temperature.
... and don't use your side stand only use the centre stand, using the side stand will allow engine oil to leak down into the cylinders which is bad for the motor and will make a nice smoke screen when you take off.