Make it yourself!
Cover your engine with plastic sheet.
Use blue foam from Aircraft Spruce.
Place the foam in the general shape you want your cowling, secured to the engine with wire, try to get it fairly close to the
mounting points already used. Cover the mounting franges with plastic tape.
Now use a spray foam between any adjoining pieces and at the edges.
Use a hack saw blade turned sideways to get a better shape. You can get a very nice surface using another piece of foam to "sand" the surfaces to the shape you want .
When you get really close to what you want, get container of wall joint compound at your favorite builders supply store.
the joint compound is used to fill the surface of the foam, a fine grit sand paper will aid in making it very smooth.
Then paint the surface with water-base paint. When dry spray mold release on the surface.
Get a quality fiberglass resin, (Aircraft Spruce comes to mind.) and apply the bidirectional fiberglass to the mold, the number of plies is up to you, but using the parts of the original cowl will offer some guidance.
When cured you can pull off the foam mold off of your engine. Gasolene will desolve the mold leaving you with a very accurate product of your mold.
roughen the out side of your cowling and apply amixture of microballoons and fibrglass resin, heary on the micro balloons (it will be lighter this way). Sand smooth and paint.
This is a bit of work, but you might develop the ability to make an entirely new cowl. I bult an enire airplane using this procedure for many parts.