I've done both plastic welding (as per
http://www.youtube.com/watch?v=iv_SpbqyQvI to give you an idea) and also fibre glass on the inside and a sweep over smooth over bodyfiller on the outside. If you prep well and take your time then you can get a good result but like mjc said painting it might look slightly out.
If I was you I'd take the fairing off choose your weapon of choice, repair the crack and respray the whole thing your self.
Or if it's an expensive bike/you want to/you **** up the spraying take it to a pro to spray - I imagine it'd be somewhere between £50-£100 for the whole fairing.