I was thinking of something simpler than a planetary gear. Imagine a 2"/50mm PCD gear on the engine main shaft with the teeth pointing inwards. This can be made very strong and possibly fastened on to the first drive gear. Then imagine an alternator, mounted onto the primary chain cover in such a way that sliding the primary cover on slides the gear on the alternator shaft into mesh with the larger gear. The alternator shaft gear would have a gear with outwards facing teeth with a PCD of 1"/25 mm. Please note that these values are for example only and the gears might need to be made larger for strength or other reasons. This would rotate the alternator at double the engine speed and have the added advantage that the alternator and its wiring are fixed to the primary drive cover. It would also allow the alternator to be moved to the rear a little this getting round the possible restriction of the diameter of the original crank case casting at the front of the primary drive. I have not drawn this up with CAD but until I do then I cannot see where any problems would lie. It might be thought that driving the alternator gear from only one side might cause problems but that is exactly what happens in gearboxes and on the original dynamo drive. I am happy to exchange ideas and accept criticism.