It is generally possible to replace the stock halogen bulb with an LED bulb in the headlight of the 2018 Honda Unicorn. However, it's important to ensure that the LED bulb you choose is compatible with the electrical system of your motorcycle, including the auto headlight feature. LED bulbs typically have lower power consumption than halogen bulbs, so they may not trigger the auto headlight function properly. It's recommended to consult with a professional or refer to the motorcycle's owner's manual for specific guidelines and compatibility information before making any modifications.