I think I understand your dilemma. Now that you’ve cut a hole for the engine and head you can’t get the cowl on without making a much larger hole?

I have Chipmunk built by another modeler with an OS 90 and the only way to get the cowl off is to unbolt the engine and remove the two together. Same principal installing the cowl, I stick the engine and muffler through the hole in the cowl and slide both into position simultaneously.