Iíve had really good luck with a product called Mat 7 Plastic Repair Kit, which is a type of cold weld repair. It is a powder with a small bottle of resin that is applied using a needle-type applicator. After forming a drop of resin at the end of the needle, you dip it into the powder and then apply it to the crack. I used a knife to make a v-shaped groove in the plastic, filled it in and the fix has lasted for years.
The product is made in Japan. I bought it at a booth at one of the Cycle World Expos years ago from a outfit in Redondo Beach, CA. Don't know if they are still around or not. The ph # on the brochure is 310-316-0937, and there is another number for Riverside, CA 909-637-0452. The website URL doesn't work anymore, but it looks like Eastwood is selling the same stuff as Plastifix.for $29.95.
ABS cement from Home Depot et al works good for gluing ABS fairing pieces together. Added bonus is that the glue is black in color.