update: once the hex nut 7 is removed, you should be able to slide out the assembly. This will need some patience though. In my case I drained the gearbox oil and removed the gearbox support bracket as well (use some other means to support the gearbox).
Once out of the gearbox, you can remove the angle drive from the assembly and replace it. While at it, you might want to consider to replace the sealing ring (12) and washer (13) as well as the O-ring (5). Removing 12 and 13 is tricky. Prying out seems to be the hard way. A simple (DIY) pulling device does the job more easily. Smoothen the surface of the assembly as well as the cylinder in the gearbox for a more easy assembly. Note the hole in the assembly which receives the pin from hex nut 7. Tightening torques can be found the workshop manual.
Looking back, the angle-drive was most likely damaged by the very heavy movement of the speedo mechanism. So, check if you speedo turns smoothly. Or better, once you notice unstable speedo readings: make sure it is not caused by heavy movement of the mechanism.
For a similar post (in Dutch) see: http://www.deklassieke912.nl/forum/s...ersnellingsbak