I have had some success repairing cracks on my ZX9R fairings with a 35 watt soldering iron (needs to be low wattage or you set the workpiece alight or it runs out of control) The soldering iron needs a thin electrical screwdriver style soldering blade. (avilable on e bay for less than a tenner)
If there is any oil or grease in the area of the crack this needs to be flushed out with brake cleaner or acetone (although acetone will remove any transfers/stickers and may attack the plastic depending on material used for the fairing or mudguard). Wash with mild detergent solution so no contaminents get in the "weld"
Wear a good quality spray painting mask as the fumes emitted are almost certainly toxic
Let the iron get as hot as it will get then lay the blade across the crack and press into the plastic at an angle of about 45 degrees to the perpendicular for about half the depth of the plastic, remove the blade, move it about 2 to 3 mm further along the crack and repeat until you have stitched the length of the crack.
Once stitched on the inside using a fine rotary file to trace the crack on the outside to form a v shaped channel about a third of the thickness of the parent material along the length of the crack. Then fill with flexible plastic filler (used for repairing car bumpers), rub down, prime and respray.
https://www.youtube.com/watch?v=gRCMIDILfEI