For planes like this, I look at online pics of the full scale model, of take plenty of pics when I'm at small airports. You can cut slats, make vent scoops, or you can make little vent plates out of plastic, balsa/covering, etc. If you look closely, you will see places where the cooling flow exits on the full scale.
If you Google "airplane exhaust vent" and "airplane vent" and hit the Images button, you'll get some ideas.
For a scale look, I'd probably go with a slatted vent cover on either side of the cowl, placed low and white in color. I'd also go with a 'backwards' air scoop on the bottom of the plane. I'm working on a Beechcraft Bonanza which has similar features: