You can sometimes repair cracked plastic with fiberglass. I rebuilt a 1981 Honda Goldwing fairing that way.
What you do is sand or grind out say 3" on either side of the crack; thin the material down so that if you glass on a piece of fiberglass cloth, it does not stick up noticeably higher than the base material.
So for example, say your plastic in the end caps is 1/16" thick. Use a sander and sand a strip 3" wide in the plastic on both sides of the crack down 1/32 of an inch. Then, mix up your resin and hardener and saturate a piece of glass cloth and lay it over the crack. Once it hardens you can sand it a bit (not damaging the cloth) to smooth it up. Then "repaint" over the area with another layer of hardener infused resin (commonly called a "gel coat"). Let this cure a few days, and then sand it. You can make a good repair that way. You have to make sure that you sand the plastic enough to roughen it good so that the resin will adhere to it. And if you can, it's best to put a layer of glass cloth on the backside (toward the aluminum shell) of the crack too. That way you cover it from both sides.
I did a bang up repair on my Goldwing fairing that way. It's a lot of work, but you can turn out some nice results. If you have a boat repair shop anywhere near you, go and talk to their guys. They can probably steer you in the right direction.
See you on the road!