Recently completed building my 6volt electronic ignition system (fits in mag/dyno) for my B31, cost me about £60 and so far so good, also totally reversible.
If your interested in building your own take a look at my video on how I did it, the idea came from https://pre31.wordpress.com/category/magneto/ and looking at pictures of the Thorspark system.
My video:
https://youtu.be/fAHYv28y9mE