I suggested additional front cooling openings back on post #49. I thought the combination of the open muffler tunnel (it is open unless the O.P. has covered it) and the exhaust exit hole would be enough but it wouldn't hurt to open up the exhaust cut out in the bottom of the cowl. It won't be seen even in the air and in addition to helping the hot air exit the cowl it will make it easier to take the cowl on and off. I often make that a "U" shaped cut out with the back of the open area all the way to the back edge of the cowl (no connecting "bridge " across the back of the opening"). It doesn't seem to weaken the cowl as the mounting points are on the sides. As John points out, cooling air flow is important to the proper operation and longevity of the engine. So after looking a several example Cub cowls, make the intake slots and oil cooler hole as big as you can and still look "right", then do open up the bottom air exit area some just to make sure. P.S. I told you John and I disagree on fueling valves!