has anyone ever tried or mounted a coolling fan for the engin ?

You'll need to do a DC conversion with a rectifier and battery to run any fan from another ATV or bike.
But it really is unneeded if jetted correctly or slightly rich
 
Yep, 240/260 sounds lean to me, unless your @ 3000'+ elevation?
better get some plugs and 270-300 main jets and do some plug chops !