The Zenoah ZG20 EI has an ignition system that operates off a 1S LiPo, which is 3.7 Volts.
Does this work with the sparkswitch, or is it below the limists for the input voltage?
It should work, but I didn´t test it!
Is there any difference between input and output voltage here?
if you will have problems with amplitude of signal from ignitor, you can use one newly developed device, originally developed for OS MAX ignitors. This device modify input signal from min 1.2 V to 5.9 V. Both sides are isolated by optocoupler, therefore it's safe if you are using different supply for hall sensor, directly from ignitor.
Have look into Spark Switch Pro and OS GF-30 RPM
Have a great day