I've had to repair plastics many times. Cracks, as said, drill at both ends of crack, then repair. Then what I've done to fix is to make a mixture of MEK (Home Depot or similar) with ABS. ABS is black but can be painted. Essentially take about a 1/4 cup of MEK and mix in pieces of ABS. I literally took an ABS pipe and started grinding it with a bastard (yes, that's the name of the tool) to get ABS dust. Mix it in until it gets to the consistency of pancake batter and it's good to go.
I mix and store in glass - an old pickle or jelly jar is perfect. I've used that batch with many different projects, not just the original.
The mixture dries fast, is strong, sandable, paintable, and it bonds and effectively becomes part of the part you are repairing.
Also, here is a method used for Apache trailers but can be applied anywhere.
http://www.apachepopups.net/OldSite/pdf%20files/abs.pdf
Finally, I also understand Harbor Freight might have a plastic welding kit but I have not seen it.
The usual warnings apply - only work in very well ventilated areas. Work with on test materials before going to the real rodeo. A little dab'll do ya.