With this type of pull start, there is a spring loaded pin that projects from the center of the hollow crank pin.
From the pic it looks like either the pin is pushed all the way in and stuck there...maybe the spring has stuck or it's missing....OR the pin has sheared off flush.

I have several of these, and worked on many others...it might be as simple as un-sticking a gummy spring. Or you might need to get another spring, if yours got lost...they can go "poing" and fly out for several feet, and get lost quite easily...(voice of experience here )

Use some fuel on the crank pin to try and dissolve any sticky residue, and work on the pin, wiggle and jiggle it to loosen it...then try and remove it w/ a magnet or forceps, needle nose pliers, etc.
Watch for that little spring!