Check out www.urethanesupply.com
(USC) for a multitude of plastic repair products and precedures, including the methacrylate PlastiFix, same thing as Plastex above. Wiki says methylacrylate has an acrid odor, toxic and is primarily used to make synthetic carpet fibres so it can't be that bad!
Anyways Suzuki uses rigid black ABS with varying additives for most fairing and plastic bracket parts (not for 85/86 front fenders they are polyurethane). I have had poor adhesion and cracking problems with my ABS fairing crack repairs using methacrylate resin so I usually hot-weld it instead for more toughness so you can bend it like the original without busting it. I have had success w/ methacrylate repairs where it doesn't need to be very flexible - reattaching fuse-box tabs, tail tabs, etc. That said, methacrylate can be used to repair cracks if you make & fill a shallow V on both sides like the OP said, maybe make it thicker on the back to help or put some SS or fibreglas mesh in the glue.
Here's USC's recommended repair methods for ABS:
1 - hot weld (hot air or electric heat gun w/ ABS rod and/or sheet stock)
2 - Methacrylate (for rigid repairs)
3 - cyanoacrylate (ie crazy glue, very rigid repairs, no flex)
4 - 2-part epoxy (Bondo kit etc, can handle some flex but sands poorly)
Note you must clean/prep the ABS extremely well (solvent then soap then acetone) if you are using a glue as they must bond to the existing surface. You can be a little less finicky with hotwelding as it burns volatile crap off and melts new ABS into the existing ABS in the process.
No connection with USC but I use their stuff. I have lots of broken fairings.