An electrical starter system and construction kit for retrofitting Harley-Davidson motorcycles built from 1936 to 1957 with a mechanical starting device. The starter system includes a clutch device that can be coupled to the transmission main shaft for transmitting rotary motion having at least one starter gearwheel, a clutch hub, a clutch cage that can be or is attached to the hub for transmitting a rotary motion and an electrical starter motor with a starter shaft pinion that can be driven by the electrical motor for engaging the starter gearwheel. The starter gearwheel can be mechanically coupled to the clutch hub for transmitting a rotary motion to the hub. Preferably, a gearwheel/hub adapter couples the clutch hub and the starter gearwheel. The adapter is or can be coupled to the starter gearwheel and the clutch hub and is or can be coupled to a transmission main shaft.