SdKfz 2 "KettenKrad". Or Kleines Kettenkraftrad. Made by NSU.
Please change name: "Sd.Kfz.2" should be put as extra info, since it's only the internal army designation and therefore not part of the model name.
It should be a BMW R75, but it obviously is not (look at the shape of the valvecover). It looks more like an IMZ-Ural 750