Just a note to let you know the "buzz coil" ignition module kit, spark plug & hall effect sensor are installed and working well on a hit & miss engine that I have just finished, it's based on a David Kerzel design.
I am using a 2mm x 1mm magnet set in the cam gear at 0.5" radius which works very well at the low speed of this engine. The hall effect sensor is epoxied onto an arm that can pivot around the cam gear shaft as suggested in your notes. That change to Mr Kerzel's design made setting the ignition timing extremely simple and precise. The coil is a universal ATV coil and is the only thing in the ignition system that has caused any problems. The first one had a loose primary terminal.
I did successfully bench test the spark saver circuit with a second hall effect sensor but have only installed the single sensor circuit as battery capacity has not been an issue. I am using a 6V 4.5ah rechargeable lead acid battery (AGM type) which I haven't had to recharge yet and cost very little more than a pack of 4 AA energisers and case.
One low cost change that I suggest you could make to your kit is to use an 8 pin IC socket that is soldered onto the PCB first and the 555 Timer IC just clips into that socket after everything else is installed. It removes any risk of exposing the 555 to the heat of soldering and allows easy replacement of the timer if ever required.
Thanks for the quality product,
Bundaberg, Queensland, Australia