From looking at the photos I cannot see anything missing.

Normally the silver pulley on the engine would turn all the other pulleys to drive the machine forwards in the same manner i.e. clockwise rotation for all the pulleys and belts etc. For reverse mode the friction disc behind the red pulley would mesh in with a groove on the silver pulley therefore clockwise from the engine pulley becomes anti-clockwise drive for the gearbox.

It looks to me as if the friction disc may have worn down and may no longer be meshing in with the silver pulley, or else the replacement silver pulley is of the wrong dimensions and doesn’t touch the friction disc.

The lever with the black knob and silver button, top right in my photo, links to the back of the red pulley – there may be a slight variation between some of the machines though.

I no longer have my Bolens to check but have some photos.