If you don't find a replacement, you can make one. Attach a block of styrofoam or balsa onto the firewall and work it into the desired shape.
Next, mix up a plastic bowl of plaster-of-paris and push the shaped block into the mixture, holding it there until the plaster sets.
Then, pop out the blank and you have a cavity to lay up a fiberglass cowl that will fit your fuselage perfectly.
Other hints... If you used balsa, seal the surface and coat it with paste wax to help you remove the blank from the plaster.
... If you used styrofoam, just pour a bit of solvent (gasoline will even work) onto the blank and it will disintegrate, leaving the cavity for a mold.
I have used this method many times and sometimes it is the only way you can easily create some shapes of cowlings for scratch-built models.
Johnny AMA 77741