New cowl's are always an option I think that you would be fine if you left the front of the cowl alone. For the bottom, yes I would make the hole the same size as the one you have right now.

I really wanted to make a nice oval shape on my front of the cowl so that the plane looks more scale-ish from the front. And then I think I got carried away with the fake twin engine picture on the baffle [X(]