I suggest the following:
Clean the INSIDE of the cowl thoroughly with alcohol (acetone WILL attack and damage a fiberglass cowl and will also damage an ABS cowl )
Using some 2 oz fiberglass cut patches to cover the areas to be repaired and to extend about 1" beyond any damaged area.
Do this step ONE PATCH AT A TIME and read completely before beginning. Place a patch in place and smooth dowm with your finger. Next put THIN CA on the patch. If the patch needs to be pushed down more use a metal rod with the end rounded to push down the fiberglass patch. If you need to push the patchdown you will need to work quickly since the thin ca will harden the fiberglass within seconds.
My experence has been that fiberglass resin has a couple of problems. If the cowl is fiberglass and you use the wrong type of resin you may be lucky and just have a mess, OR you can damage the cowl. Also, resin does not adhere as well to ABS as thin CA (at least for me it doesn't
When the ca has dried (typically about 20 minutes ) proceed to the next patch.
When all the patches are in place and dried, clean, then sand the outside of the cowl, then clean again.
Now you can add filler as needed to fill any cracks, etc.