I use visor rain repellent by these people
storm water proofing.
It's not perfect, but it does do something. It doesn't really help with the build up of spray/mist at slow speeds, but turn your head sideways at 60+ and all the water is just whipped off. It wears off pretty quickly as well, you need to reapply about twice a week in bad weather.
I bought it from the local Kawasaki dealership.
I'd previously tried some stuff for car windscreens that I'd bought from Halfords. It wasn't quite as good.