You could try PM'ing John here about replacement parts... ( Nitrostaff1 ).

Consider supporting the existing cowling off the plane with some clamps so it holds it's shape...

Then covering the outside with fiberglass cloth, and applying epoxy...

Do this twice, then sand the other cowl, finish and repaint.

You also could try cowls from some of the aftermarket RC places, such as fiberglassspecialtiesinc.com

[link=http://www.fiberglassspecialtiesinc.com/newprod.htm]Click[/link]

Another great suggestion from someone else here...

Make a similiar shape out of foam... glass the shape and clear out the foam...

There is also covering over foam, or building a new cowl out of balsa.... FlyRC and SportFlyer Magazines both have had articles on how to do this for cowls and wheel pants.