No answer: I'm just adding my name. I have acquired a '66 BSA A65 with a MityMax and ARD belt driven magneto. I'm getting no spark and am unfamiliar with both the MityMax and the ARD magneto. I have found very little material on them available on-line.
I'm hoping that someone will answer your question so that I can start to troubleshoot my ignition by checking the MityMax.