The main fairing body and tail cowl are some sort of fiberglass-ish stuff (I forget the name of it) that uses indivdual glass fibers and some sort of resin to hold them together.
The other body parts are ABS or similar plastic. (On K1100s.)
What I've had good luck with in the past is roughing up the inside of the area around a crack for better adhesion and then using a fiberglass repair kit. I usually rough up the surface with a Dremel and then wipe it clean with rubbing alcohol before applying the fiberglass. I often use three layers of fiberglass which is probably overkill.
Since I'm not a bodywork guy I then let a painter fill and sand the crack on the outside before painting.
Here's an example of a K75 belly pan that I repaired. Since I was having it covered in truck bed liner so it didn't' have to be perfect I did the exterior filling and sanding myself.