There was a thread in the restoration section about reparing grills and I think fan shrouds.

You need to open up the crack a little and bevel the edges. I think it said that you need to get another piece of ABS plastic and cut it into slivers. Place the slivers in a glass container of paint thinner. The thinner softens up the ABS slivers and you are left with a plastic mush that you can smear into the crack. Once it dries, you can sand down the repair and finish it off.