1) Take a lego block (or a piece of ABS pipe), dissolve it in acetone till it's a slurry the consistency of peanut butter.
2) Drill a 2mm (approx) hole at the end of the crack to stop it from running.
3) Prime the crack and 1" around the crack with acetone on the back side
4) Slightly spread the crack, and paint in some slurry.
5) Back side paint the crack with a 1mm thick coating using the slurry.
I'm doing a repair on a side cover this weekend, I'll film it - it's **** simple. I've done some destroyed fairings this way, they come out looking like new.