If the cut piece will lay flat again when pressed down, use a small amount of two part epoxy, spread it into the cut, then use two pieces of wood and clamp the shit out of the repair until the epoxy has cured. You may have to sand/file and epoxy that squishes out but at least the topsheet will be back in place.
If it won't lay flat, cut away any excess material and again use two part epoxy to fill the area. This time though you want to spread it evenly almost as if you're sculpting a new topsheet. Then again let it cure and sand/file away.
Epoxy does yellow over time so I've repaired edges that end up looking weird after a couple years. Who cares though as long as the board will still ride...