Since we are still talking, what aboutintegrating carbon fiber cloth with the fiberglass for additional strength?
You could, but I doubt you need that much strength that it is worth the relative difficulties or the extra weight. By using fiberglass, you already have a far superior strength and durability in your cowl than the original plastic one. For .40 - .60 sized planes, I usually lay up my cowls using two layers of 3.5 or 4 oz FG cloth cut into strips with some overlap. I will run about a 3/4 - 1 inch strip around the rear (or trailing edge) of the cowl for extra rigidity and for durability if I plan on drilling holes at that location for mounting screws. Strength and ease of layup also depend a lot on the shape of the cowl. Long, flat sides will flex more and are weaker than those with corners, arches, corrugations, etc.; but these are the shapes that make getting the cloth to lay down more difficult.
It also largely depends on the type of CF cloth you intend to use. Some types are very rigid and do not conform to compound curves well during simple hand lay-ups. Using an existing plastic cowl doesn't sound as if you would be able to put your lay-up under any significant pressure (such as vacuum bagging) to ensure the CF cloth lays perfectly.
If you want a little more strength, for a very minimum weight penalty, and want to keep this relatively easy, you might consider using 1/4" (or similar) carbon fiber tow in an overlapping weave pattern. I tried this on the nose sections of some of my latest warbird racer fuse molds and was happy with the results. A couple of pictures demonstrating how I incorporated this method in my last 1/10th scale (.40 sized) P-47 hand layup are attached below.