Pegaso motor issue with engagement sprocket

Good afternoon all,

since a few month I am driving around om my Pegaso. Last weekend I went on a longer trip of almost 200 km in two days with my brother (He drives a Honda C50Cup).
I had some issues with the engagement of the outgoing axle to the main sprocket on the outside of the engine. It would randomly disengage. So I needed to downshift to engage again. As well this wouldn't work always. Does anyone have an idea how I can fix this? Should i completely restore the gearbox or is there something I need to adjust?

Here some photo's of the trip!

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To solve the issue I had to completely disassemble the motor. While everything was open I changed all the bearings!

During putting the motor back together I came to answer my own question. The button is for disengaging the sprocket to the rear wheel. This will prevent the bike to take-off after starting the engine up.
Once you select the 1st gear you'll pull the ball bearings outward in the axle of the sprocket. The outgoing axle is now connected to the sprocket.
The reason why the engine lost it's traction is because the shifterfork was not properly aligned. This caused the internal expansion balls not to expand all the way to lock the gears in place.

During rebuilding I as well noticed the oil pump piston was broken, so I remade this on the lathe. Unfortunately my measurements were off and tolerances too high. Got my cilinder stuck after a little drive due to little oil pressure and too little lubrication.

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