There are few prices listed, which often means they are frightening, but these are mechanically reasonable options for motorising a bike.
http://www.bavariabike.de/estart.htm
http://www.bavariabike.de/saxclastd.htm
http://www.bavariabike.de/mozzi.htm
http://www.bavariabike.de/angebotm.htm
http://www.bavariabike.de/estart.htm
http://www.bavariabike.de/saxclastd.htm
http://www.bavariabike.de/mozzi.htm
http://www.bavariabike.de/angebotm.htm