ORIGINAL: Bodge
Make sure the air outlet hole in the engine cowl is about three times the area of the inlet hole. I tend to cut the bottom out of the cowls to make sure there is enough airflow over the engine.
I would have to add to what Mick wrote.
Install baffles inside the cowl, to direct the air-flow over the engine and its head, to provide ample cooling.
Just having entry and exit holes in the cowl, will not make that air cool the engine, unless it is forced to flow over it.
This is what these baffles must do.