The problem with that idea I think, is the large diameter of the clutch drum, which would spin the alternator about twice as fast as needed. The small garden tractor alternators, such as the excellent Kubota, are intended for use on relatively very slow revving diesel engines, with windings to suit. I know that the late Don Hewitt fitted one to his Scott special, and Barry Jackson now owns the bike. Might be worth talking to him about it. I do know that Don rewound the alternator for use on the Scott, so there is a lot of work involved.