Yes you will want to re-enforce it...unless your model was specifically designed for a outboard motor..
True story..
I added a outboard last year on my C-27... Bought one of those extenders (long shaft 9.9 so needed it to be away from the transom to get it up and down)..
- I added teak plates to the mount of the extender...
- Inside the cockpit portion of the transom I cut out a same rectangle and inserted another set of teak blocks...
- Then I used plywood cut only slightly larger than the other two..
- Bolted it through ...
It the motor flexed the transom anytime I was at cruising speed or above...Enough to think twice about seriously using it full-time...
Lesson I learned - cut a plywood portion for the backing the length and height feasible of the interior of the transom to buff up the entire area...Or best - if you have room - glass in a few more layers to stiffen....
but working in that area was a tight fit...neither of those two options were feasible until after the fact...