Moped Army

See if I recall this right: that "switch" or driver mechanism is spring loaded and the spring is held into place by a washer secured by a circlip. You can remove the driver by first removing the small circlip with a screwdriver, at the same time pushing against washer, and then release the washer and spring comes out. Keep those parts in a plastic bag or something. One possibility is that the spring is broken,
and it therefore does no longer exerts the force necessary to fit the driver to the sprocket.

The sprocket, has got two "teeth" perpendicular to the sprocket disc that fits into the driver. The driver transfers the power to the chain this way. Are those "teeth" ok?

You disengage by pulling out the driver and rotating it 90 degrees. Likewise, do the reverse to engage.