Firstly you need to release the jam, this can be done by forcing the center cam back with a screw driver. I wouldn't recommend this! Just slacken off the 2 socket head screws on the ends of the clutch shoes, these are the SG2.6x12.
Now reassemble, try to get the SG2.6x12 to the same setting, as this determines when you change into second. Reassemble the mechanism onto the shaft. Screw the center SK4x5 in until the shoe T0234 starts rubbing on the clutch bell T0236. Repeat this for both SK4x5. Now unscrew both SK4x5 about one turn, and you are done.
The ball t0239 needs to operate on the flat on t0233, I suspect that your clutch shoes have worn down and the ball ran off the flat part, and ran onto the rounded part.
The 2D picture in the sidebar gives a better understanding of how it works
