Forget everything but this. Buy a cheap soldering iron for around $10-$15 with an assortment of tips at the hardware store. Match the broken piece up to the area it came from. Heat the soldering iron and tape the piece back into place on the visible side in order to work from the inside. Using a broad tip with the iron at full operating temperature. Slowly and gently touch the seam/crack so that you "sweep" the molten plastic over the cracked area. You can use a circular motion once you have effectively covered the crack with molten and now hardened plastic. Do not get too anxious and JAB or gouge the plastic with the iron, just try to achieve as uniform a repair as possible on the inside of the piece. This should give you the results you desire. If you are a perfectionist you can experiment with repairing the painted side but I would advise against that. However you can use rough sandpaper 80 or 60 to rough up the painted side if you intend to have it repainted. Once it is rough you can go ahead and use the iron on the crack and repeat the melting, sweeping, circular motion until you have made the repair indistinguishable. Then sand it again, prime it and topcoat. It is a very easy process and once you do it one time you will probably want to go into business doing it yourself because it is such a piece of cake.
Forget about fiberglass bondo and plastic body filler because when it comes to motorcycle plastics you can easily accomplish a very aesthetically pleasing repair.
