-- Last Updated: Oct-23-12 4:41 PM EST --
but I share the same concerns about the ABS being brittle that others have, and would be reluctant to use it on whitewater.
I would use G Flex epoxy rather than a methyl-methacrylate like Plexus simply because it is much cheaper. It bonds to ABS well. Mixed with a thickening agent like colloidal silica, it can be used to fill voids and fair uneven surfaces on the hull. It also wets out fiberglass and aramid cloth well.
I would try to fit the pieces of broken Royalex together like a jig saw puzzle. Try to secure a stiff piece of cardboard covered with Saran Wrap to the outside of the broken hull to maintain shape. Clear plastic packing tape can also be used to hold the approximated pieces together. Thicken the epoxy with colloidal silica to bond the broken pieces together from the inside and use multiple applications to fair the broken hull surface. G Flex can be mixed up in very small batches and sands well when cured.
When you get the interior as smooth as possible do the same thing on the hull exterior and fair it as best you can. I would apply a single large sheet of aramid to the inside of the hull extending at least 2" beyond all broken edges. You might want to use peel ply over the aramid to get a smoother edge. On the outside I would use a single layer of S 'glass again extending at least 2" beyond the breaks, but try not to have the edges of the fiberglass line up with those of the aramid.
You could use more layers, but I suspect this repair would be as strong or stronger than the remainder of the hull. If you wanted you could cut strips of S 'glass to bond over the joined breaks and feather the edges of those strips before covering the whole with a large sheet of S 'glass.
Feather the edges of the S 'glass well and fill the weave with enough applications of epoxy. Paint it over with red Krylon Fusion spray paint, and if desired, you can spray over the aramid with some flat grey spray paint as well.
You can buy a 32 ounce kit of G Flex epoxy from Sweet Composites for $56 and I think it would be enough to do the repair: http://sweetcomposites.com/WEST.html
Three ounces of aerosil (colloidal silica) will set you back another $4. A piece of 5 ounce per yard aramid 36" x 58" would cost another $27: http://sweetcomposites.com/Kevlar.html
and a piece of 6 oz/yd S 'glass 36" x 60" would be another $19.50: http://sweetcomposites.com/Fiberglass.html
A 36" x 64" piece of nylon fabric (peel ply) would cost $9.50: http://sweetcomposites.com/Polyester.html
So far you are up to $116 less shipping. Adding costs for incidental supplies like disposable gloves, sandpaper, tape, spray paint and shipping costs will put you around $150 or so. To me that would beat taking the boat to the landfill.