I've been fixing parts like lately with a cheap FHT plastic welder and the stainless-steel screen mesh that comes with the kit.
For a cowl insert, you remove the insert. (usually screws from the inside, but sometimes they are plastic welded)
Then, from the BOTTOM you cut out your mesh screen to cover the damaged area. Lay the part on a towel, and lay the mesh over the cracked area. Then you use the welder to heat the mesh and melt the plastic below it. As it softens, you push the mesh down into the plastic, and keep working the area until the mesh is completely buried in the plastic, greatly strengthening it.
You could repeat from the top if you need a lot of strength, but not usually for an insert.
Then, take the welder iron, and groove out the cracks with the point and then smooth it out immediately after to weld the top of the cracks closed. You may need to use filler rod to build up the area a little
All of this takes maybe 15mins. If you did a good job, sand it smooth and paint. Other times, it's easier to sand and wipe a little body filler over it makes it all go away. Prime and paint as is usual.
If a top cowl is in really bad shape, I usually look on Craigslist or FB marketplace and find a new cowl. Very often, it's well worth it.