I would again suggest to replace the bonding strap from a better connection on the motor like the intake stud to go to the lower external coil stud. If the bonding strap is sketchy your electrical common (ground) will be via your control cables (decompressor, throttle and choke) which is not the best connection.
I can not see the bike so I can not tell if there is a connection which has been knocked loose or even broken. Motobecanes have brass contacts on the electricals not copper so they do need to be cleaned up from time to time. Could be a combination of all of that together. Make sure the zener diode has a good tight connection electrically and it is on the frame goodly.
If you disconnect the accessory wire from the back of the genny and attach a volt meter on low volt AC you should get between 4 and 8 volts just above idle and 8 volts with the rpm up. If not then you would want to change the zener diode or substitute a 6 volt regulator.
There are a lot of wiring diagrams over at Mabecane Forum, including 6 volt, perhaps those will help -