Rebuild boat

Hello all!
I recently bout a 13 1/2 foot fiberglass boat. I have ripped out the old floor and transom. I am re doing the transom now, but I have a question that I was hoping someone on here could answer. I am thinking about putting about a 20-25 hp on it. How thick should the transom be, to hold a motor of this size? I glassed in a wood so far that was 3/4 in. Do I need more?

 

You need 1 1/2 of transom and stout knees connecting the transom to the sides and bottom plus metal protection to take the motor clamps. Saw a too-big motor rip the transom out of a boat once and take off on its own.