deejay
Radio Shack, here in the U.S., used to sell a little 2 inch diameter horn. It is ment to be used only as a warning device on a machine, not for a vehicle. O.K., it's just a loud buzzer. But it will pass as a horn. Lots of room under the forward portion of the fuel tank. If the center stand is removed there seems to be some room in that area for the stock horn if layed flat. In any case, never had the law ask about a horn. Have got away with that for years.