I've been thinking about the thermosiphon cooling idea. Cutting off all the fins but the top and the bottom, then using aluminum brazing rods, brazing a sheet aluminum water jacket. Obviously fittings for a hose. Needs to be included.
Pumped systems need two hoses to return the pressurized fluid back to the pump so only one may be necessary.
I like the idea of a float valve but it justs adds complication. One concern I do have is when does the rider know to top off? A dip stick works but that's not happening while riding. I have seen transparent fuel lines used as a measure for fuel tanks without gauges. Maybe just make the tank with Lexan and keep it dead simple