I've installed a lower radiator hose heater in one of my DK45se tractors.
Radiator hose size: 44mm OD, 34mm ID
Kats 16600, a 600Watt 1.5" in line heater, Slighly larger than what is needed, but its the best fit, and fits fine. ($37)
Plugged in over night, from about 8pm to 8am.