I've had a great deal of success repairing fairings with old bits of ABS. ABS dissolves in acetone which is freely available through ebay. I've repaired mounting lugs with it, fixed cracks and even filled holes with it. For cracks, first drill a small hole at the ends of the crack to stop it spreading further, vee out the back of the crack, fill with a creamy paste made from ABS dissolved in acetone, and reinforce the back with fibreglass matting soaked in the paste. It works well because the acetone in the paste softens, then bonds to the surface of the repaired object. I sourced sacrificial ABS from a couple of old bits of fairing laying about in my garage, and also from the front cover of my knackered spin dryer, you might be surprised just how much stuff is made from ABS. A quick test for ABS is to wet a bit of cloth, or tissue with acetone and rub it on the plastic you're testing, if it's ABS, it will start to dissolve and will mark the cloth with the colour of the plastic.