RE: Is this enough engine cooling?
The side hole for the head is not needed and you will get no ram air effect at that location. In other words no cooling help. I would however use a head hole for plug serviceing. With a 135 degree mounting it is important with the added possibility of hydro lock with careless handling.
This is a full pressure cowl and putting all the exit area around the muffler header area like you described will be a major heating liability. The air must be allowed to flow past the engine then out. Doing as you described will dam up considerable air behind the engine and become an oven.
Here is a picture of the outlet on an airplane that I use up to 40% nitro in and notice Its also a 135 degree mount with the outlet aft and favoring the side behind the cyclinder and a slanted baffle to smooth the outlet flow. No air can dam up in this way. This works very well and on this one the outlet area is around 2.5 times the inlet if you discount the cylinder hole, through which very little air goes in or out at that point.
John