If you want to add an Outboard Engine to your plywood boat you should pay some extra attention to the transom.
In the original plans for most boats the transom is just too thin to hold an outboard engine. It would hold the engine at first but if you accellerate, the transom could break and you may sink the motor or even the whole boat.
For that reason I doubled the transom over the whole wide with an piece of hardwood about 20mm thick. Once again, the hardwood was glued with epoxy on top of the existing transom.
Support Triangle added to support the transom for an outboard engine
I also added a support triangle inside of the boat which supports and connects the lower part of the transom to the bottom of the boat.
How big the engine?
On my "Dive Boat" (Fishing Punt) I put an 8 HP Yamaha Engine but it would also work with 5 HP up to 15 HP. Bigger than that I guess the engine becomes too heavy for this kind of boat.
>> See the Video here how my boat ran with the 8 HP Outboard Engine
The 8 HP engine I used, would not make the boat to glide over the water but it was still fast enough for a tour, even with 5 pax inside.