Sadly, as far as I can tell, unlike many other states CA does not make registration cheaper for antique or historic vehicles, so not a lot of point in getting them.
The "antique moto" (pre-war) plate is different than "Historic Vehicle" and I am pretty sure motos can get the historic vehicle tag (1922-25 years old) like a car since the application just says "motor vehicle" but I can't think of any reason you would want one. Same registration cost while limiting the vehicle to "exhibitions, parades or historic vehicle club activities"
Edit: Wrong about the historic plate for motos, but still not sure why anyone would want one.