Bryans repair advice is okay, (he should know, he reinforced his boat with it), but if the ABS is the sort that dissolves in acetone, you can also repair small cracks by "soaking" a piece of scrap ABS onto the crack with acetone, welding it to the hull.

Test this on a scrap piece before applying it to the hull to avoid buckling!!

Other ABS types dissolve very nice in ethyl-acetate, it's trial and error to find out what works best.

Regards, Jan.