My Kawasaki Bayou 220 (1989) alternator puts out 45VAC, and it goes through a regulator/rectifier (nothing else) and charges the battery. I wonder if you could intercept the alternator output (mine has 3 windings) and use a motorcycle regulator. You might have to install a small motorcycle battery or gel cell (alarm system battery) to filter the output. You would need a decent wiring diagram showing the current bike setup and the setup to which you'd like to migrate. My system can theoretically put out 13a at 12VDC and is protected by a 20 amp fuse.
Colo Springs, CO