First thing first; I don't own a CBX but I can give you a few pointers; to put things into perspective.
The thing is; it's generally not a good idea to expect miracles from a 30+ year old motorcycle. So much can wrong with such aged machines and if it runs then...
Well, that should be enough to make you happy. Don't get your expectations high!
Besides, CBX has FOUR carburetors and 4 cylinders; each having a displacement of 187cc and if it's returning ~20km/l then I'd say it's all good! And to make you feel a bit better; allow me to mention the fact that my little Yamaha YB100 2-Stroke barely returns 35km/l.
So... Don't get paranoid and enjoy the ride.
If you want to add a fan then that's fine but remember; this is an OLD machine which is at the end of its lifeline.
I can't stress that enough! That thing is WAY beyond its prime, my friend! So don't feel so bad if it gives you minor problems every now and then. Be prepared for that, alright?
Hope it helps!