Heres something I found in an article about fender repair.
Fix ATV Cracked Plastics
If you haven’t taken care of your ATV plastics, they could dry out over time and eventually crack and break. Or the more likely option, you hit something didn’t you? That’s ok, cracked plastics can usually be fixed without too much trouble. If you don’t want to spend the money buying new plastic parts, you’ll need to do it yourself.
For small little cracks you could try using the JB Weld I talked about above, but that only really works for cracks less than a few inches. But, you probably really messed your toy up bad, and for that, you’re gonna need something better.
I recommend using what’s called a hot staple gun. Like this Astro 7600 Hot Staple Gun Kit found here on Amazon. This thing is amazing and I’ve used it on more than just my ATV. It works by heating up these specially designed staples it comes with. The staples get set into the plastic making a nice strong bond, but still allowing the plastic to be flexible.
You just line up the pieces you want joined, and use the staple gun to staple them together. The staple is heated to it can sink into the plastic a bit, making a super strong bond. I always do the stapling on the underside of the plastic, that way you don’t see the staples. If you use this method, you will still see the crack on the upper side of the plastics. You can then use the JB Weld to fill in the crack, and then sand away any excess making a nice smooth surface.