I have used Polyweld before which melts the abs plastic surfaces and they join (~£3 a bottle). It takes a few seconds to firm back up so you need to hold it in place and it also needs a good join to work, it won't bridge gaps. Often abs panels bend slightly at the fracture point so this method isn't really applicable in most cases. Some people claim acetone does the same thing but I have not tried that. It is also quite picky on the materials used. The radiator guards on the fazer (for example) it doesn't touch that at all, it seems to have some sort of fibre in it, maybe to raise its temperature range...
Regular glue won't work as it doesn't really stick to plastic (not much does). You might find some specialist glue or bond that works but after many a beer token wasted on these I always use an iron now. It works - everytime. You can get "kits" that have a special heat gun and nozzle for heating abs rods but to be honest its easier to use an iron and much cheaper - people trying to cash in on proprietary kit. The variable iron I have was £25 from CPC a few years ago. Gas ones are not bad but you have to take care the exhaust does not melt anything.
If its a permanent thing I would get some abs scraps (from a breaker or find some cheap abs tubs at a home bargains store) and fashion it into one piece by soldering it up.
Saying that a good bit of duck tape can sort many ills
If the presentation side needs filling/sanding then you can either overfill the crack/area with abs and sand it back or buy some bumper filler.
