If the inner skin is undamaged, I would have cleaned the broken foam away, prepared the outside for repair by sanding away paint/gelcoat down to the glass and filled the hole with thickened epoxy (peanut butter consistency). I would have added 2-3 layers of thin glass to the outside. The outside should be prepared to recieve the glass, by beveling the area, so the first glass patch just covering the gash + 2-3 cm, and then incrementally larger patches. If you are careful when beveling the outside, very little filling/sanding need to be done afterwards.

When you have filled and layed down the glass, apply a piece of e.g 6mil (not millimeter) PVC plastic, and the surface will come out quite smooth after hardening. If you are careful and shape the outside 'freehand', you dont need a mould off the other hull. If you want a perfect job, you must make a mold off the other hull first..

I have had the same damage done to my Marstrøm Tornado, and this is how I repaired it (it's still in one piece). The are a your damage is in, is not a highly stressed part of the boat..

A last tip. When preparing the glass, place it between to pieces of clear PVC or other thin plastic with the glue. This makes it easier to wet out, and handle, before you apply it to the hull.