It doesn't use a zener diode it uses a solid state voltage regulator but yes you can use one. This regulator adjusts the current in the field coil and therefore regulates the output of the three phase stator to match the battery charge condition. The later Honda bikes use an integrated rectifier and regulator, the regulator parts being a solid-state equivalent of the electromechanical separate OEM item. Regulator-rectifiers are cheap on eBay, just check its been tested if its a used one.
Ash
