That will be a tough repair.  I think you are right to consider making a mold and get a whole new section to replace the damaged area.   I don't see how you can use the other side of the boat.   I'd try to find another good identical boat,
and make a mold off it and them make the repair section from the mold.   It won't harm the boat to make a mold if done right.    Replacing the hull is certainly a better bet for a good outcome.
You can do it the 'hard way', it will be very time consuming doing all the shaping and sanding to get it right without the replacement hull section.  my 2 cents.