Stock horns on mopeds are mostly AC, if the bike has a battery the horns are usually DC. Magnetos give out AC because the N and S are constantly swapping ends on the iron core of the coil as the flywheel rotates around it. A rectifier will smooth this out to DC and a regulator will keep the voltage to 12v or lower.
Important to not over tax a moped magneto by demanding too many watts from it. This will increase the parasitic heat inside the magneto and start to cook the insulation on the magnet wire in the coils and they will fail.