I found the following two documents very useful:
Bill Rogers of the Isetta club explains the Dynastart for the Isetta which is pretty much the same as the one used on the Bella. One little snippet of very useful information is that well functioning generator coils should be between 4.5 and 5.7 ohms. This is perfect to know if you wonder whether the generator coils are damages (note not the starter coils, the field generator coils only):
Bosch-dynastart – explained and how to test
Separately, the following documentation from the German Zundapp Bella Club is indispensable to understand the electrics of the Bella (Many thanks Heiko, extremely useful to have!). It does require a good understanding of German:
eSchaltplan-Package_DYNASTARTER_2008-01