The principal (and largest) openings in the front of an air-cooled engine's cowling are cooling air inlets. Smaller openings would be carburetor and oil-cooler inlets. Liquid cooled engines radiator inlets are almost always on a blister of some kind to accommodate the radiator (e.g. the P-51's "blister" below the wing, the P-40's "blister" behind the spinner, the P-38's blisters on the booms aft of the wing, the Spitfire's "blisters" beneath the wing, etc.). Some radiators for raceplanes were fashioned as flush surfaces of the wing or fuselage (e.g. the Supermarine racers), but were not common, probably due to the expense.
Air cooling an airplane engine is not a matter of getting fresh air in, it's getting the bad (heated) air out. The cooling air inlets don't have to be very large, but the exits have to be generous and efficient. Hence all those louvers on the sides of inline engine cowlings, and large cowl flaps in the trailing edges of radial engine cowlings. Engine cylinders are wrapped in close fitting sheet metal baffling that compress and route the air over the heads, and there's often an airtight barrier separating the ram air region of the engine from the waste region, which via cowl shape and flaps is at a partial vacuum, to increase the draft by differential pressure.