There are many ways to skin that cat, several good ones already mentioned.
Problem w/ plastic-welding, there's a fine-line between good penetration/adhesion & over-melting/distorting the plastic. Takes far amount of practice/feel to get a good quality repair - and you gotta buy a tool.
Various adhesives/epoxies can also do a good job - but they're still just gluing the pieces back together. I've used Devcon plastic repair epoxy with good results (~$3, WallyWorld), That 3M stuff PC posted would undoubtedly work great & also work as a sandable filler to prep for repainting - pricey though. Stay away from fiberglass-reinforcing resin - not good for adhering to ABS, and far better products available.
My favorite & IMO by far the best product for crack-repair is acrylic powder/solvent (I forget the chemical name). Cheap, easy to use, quick, no particular skill required - and most importantly STRONG. Like at least as strong as the existing plastic strong, since it doesen't just adhere to the pieces, it chemically fuses to them & the cured powder seems to be a bit stronger than regular ABS (made a test piece, and the plastic broke before the repair did).
The two brands I've used are Plastex (expensive), and
http://www.plast-aid.com/ - from what I have read, it's the same stuff as acrylic nail powder/solvent avail @ beauty supply places, but the cost is similar & I figure the stuff specifically for plastic repair might be a different formulation/strength so I went with that.
My bro & I are resurrecting a CBR & just last weekend repaired a tail fairing which looks very similar to the damage you posted. He did a great job with it in an hour or so with zero prior experience. I'll try and remember to post a pic this evening. It will still need a bit of filler/high-build primer to complete the cosmetic repair on the outside - but the strength & ease of the structural repair is unbeatable, IMO.