Grind the crack down from behind, get some ARALDITE on there, great stuff, put some on the front to while wet, get some fibreglass on the back of it, raw stuff, not p40 paste, grind the front down lightly, use some isopon bumper fill (filler for plastic) smooth it down, coat with regular filler, job done...wont ever be as strong as original
Unless, you get a large solder iron, and i mean a large 1", get some old plastic bits, break them up, melt the bits onto the rear and front of the crack, lots on the rear covering 2-3" space around the crack, some on the front, grind the front and apply some filler, now that would work a treat...