for calibrating the unit a transformer is needed; will e.g. a 12v battery charger do the job?
No, It must be a 50 Hz transformer (AC voltage) a charger has a DC output.
how to calibrate a motorcycle with “lost spark” (BMW R100/7)?
We can’t give support for this specific motor.
is there a possibility to take the impulse from the spark plug leads (induction) ?
No !!! It must be connected to the input from the ignition coil (from distributor)
If you connected it to the output, the whole circuit will be damaged.