Nice indeed - check both lamp bulbs, should be 6v1amp for headlamp and 12v0,5amp tail lamp, system is only 6v but the tail is lamped with a 12v bulb as the filament will be more robust to take on the vibrations.
Normally you would have the hot wire from the magneto go to the switch and to the horn. Switch to tail and headlamp as each lamp holder should be earthed to the frame as a common. Hot goes to the horn and a wire from the horn to the switch which earths to the common when pressed (ground to sound).
Usually the cycles are wired specific to the marque of the cycle and not the engine, so I dont have a wiring diagram I can dig up for you without knowing the marque and model. VAP engines were quite popular on many marques and models.
Hope that helps... Ido have a English manual for the engine if you need...