my best guess is that the outboard motor, through a flex drive, operates a reduction gear housing hidden from view by the front fender, but the drive wheel can be seen riding on the front wheel in the above photo, through a cut out in the fender. An idler wheel rolls a v drive belt via a shaft, and that spins the cooling fan in front of the radiator.
this might have been stored since the moment some mom had the scare of her life seeing her boy ride it.
Evinrude seems to have made, or licensed its name to bicycles: http://thecabe.com/vbulletin/showthread.php?16024-Finally-find-a-good-bike-and-its-a-darned-boys-bike&p=79860#post79860