The areas for the intake and exhaust are this way:

The intake area is usually sized to the frontal area of the cylinder and head. That's for sport airplanes. An engine down in a Cap 232 cowl would want all the holes in the front of that cowl to have a areas that add up to an area the size of the front of the engine's cylinder and head. One of my engine's has a cylinder about 2" wide. And the cylinder up to and including the head is about 2" also. That's 4 sq inches of intake area. I actually closed some of the big gaping holes in the cowl of a recent ARF. I left the holes in front of the engine and closed off all the rest.

Since that cowl had about 4 square inches of intake, I made sure that I had 6 square inches of area behind the engine. I actually stood off some of the cowling away from the fuselage to do this. In your case, you've got the right idea for where that is going to be and you've got to make it yourself.

Serious speed planes aren't like sporters. If you're actually trying to get the airplane up near it's top speed, you do need to consider the opening size. What a lot of people did was start with the general size for sport planes and do some test runs. They usually found that the engine wasn't running warm enough and that the opening was holding the top speed down. So they'd reduce the width of the intake opening and test again. They were matching the size of the opening to the speed they were getting and considering the problem of engine heat and tuning the drag of the opening all at the same time. It is fun to do.