Actually, first try the engine on a test stand or without the cowl. If you don't have the detonation problem, then you can be sure the engine's not getting the cooling it needs.

With ANY air-cooled engine, you have to make sure that the cooling air passes through the fins on the cylinder and head. Take a look at a full-size aircraft engine. No air going into the cowl is allowed to leave before it passes throught he fins of the engine. Baffling is used to direct the air where it needs to go. Take a look at some of the high performance homebuilt aircraft. A 180 Hp engine is cooled with two 3"x6" inlets. They can be that small because all the air going in passes through the cylinder fins.

In your model, you may have to build baffling to make sure that the air going into the cowl actually passes through the engine's cooling fins. If you can look into the air inlet and see the air outlet, then you can be sure that the air is taking the path of least resistance, and going directly from the inlet to the outlet, do not pass engine, do not collect cooling.