Graft it back together and fiberglass it invulnerably strong. This project is good preparation for making a board from scratch.
This repair method uses epoxy resin and microballoon putty. Instead of epoxy you could use polyester resin or vinyl ester resin, with minor changes dictated by the resin instructions. Remember that unlike epoxy, those other resins dissolve styrofoam. This board is urethane foam so it doesn't matter.
Stuff you'll need:
Epoxy resin and the directions to not screw it up.
Fiberglass cloth. I have 8 oz and 4 oz.
Tyvek bunny suit to keep fiberglass dust off your skin.
safety glasses, dust mask, gloves for ditto.
Angle grinder, sanding disk, backing pad.
assorted other abrasive spinning devices, sanding blocks, sandpaper