I fitted out my old R80RT with air horns, easy once you find a spot to hide the compressor. Motomail use to sell twin horns with a tiny compressor uitable for bikes. Just plugged the exisitng horn wiring into it. Sounded like a freight train passing through, scared the shit out of anyone in a car within about 10 meters and excellent fun at pedestrian crossings  👿 I'd put a set on the Dakar if there was room.