You can adapt a Lucas MO1 to fit an A single. There is one fitted to the club bike "Nigel" I cut the gearcase from the end and milled 10mm off the base to bring the spindle height down to 35mm. The dynamo drive consists of a pair of pulleys and a 100XL toothed belt. It is a very tight fit and needs a split cover fabricated to fit the points. If you can get hold of a prewar Lucas magdyno with ring cam (nearly as rare as a Miller or BTH) they are shorter and are an easier fit. I have this conversion on my A Comet. The late Bob Stafford described how to fit a Lucas MO1 magdyno without altering the drive but it meant cutting a slot in the platform to allow the Lucas fibre pinion to sit down through it. Forty Years On page263 article 45. The advantage of modifying the Lucas magdyno to belt drive is that you don't have to modify anything on the engine.
Brian
Brian