Snowbeard my friend, This what I have fitted to my bike. Combined brake and air lever, and combined clutch and manual advance and retard lever. I got these from Surrey Cycles in England. If you go to this page you can view their range of levers.
http://www.amalcarburettors.co.uk/spares_04.htm I am not sure weather you will be able to get these parts in the U.S. but as you will see there is a wide choice available.
The claimed advantage of manual advance over auto, was that on "sports" machines it allowed the rider some control over the timing. If the engine under certain conditions started to pink (ping) then you were able back off the timing to curtail the condition. I think that this was a throw back to the days when only low grade gas was available. As leaded petrol became available, and octane ratings rose the need for this facility diminished. In practice I have found that retardation is only really needed for starting purposes. Contemporary road tests in the English motorcycle magazines would refer to retarding the ignition to make for smoother running and greater flexibility in slow moving urban traffic conditions, and better idle characteristics. In practice I do not think that there is in real advantage over auto advance. The critical factor is the accuracy of your ignition timing at full advance.
For those who feel inclined, on either type of magneto, manual or auto, you could always elongated the three holes in the casting to facilitate some adjustment of the timing in the same way that you can with a conventional distributor. It would allow you to compensate for any slight error in the initial setting up, and it would also permit a limited scope for experimentation with out the chore of taking everything apart each time you wanted to try different timing settings. Naturally such adjustment would not be able to cope with settings that are wildly out, but if you want to just tweak it a bit then it would work quite well.
