I wouldn't even go to the trouble of mounting it in the hull. Too many modifications + gas motor in a confined space
The vertical drive limits options on a yak. If it doesn't weigh much, I'd make it an outboard. All you would have to do is mount a very sturdy bracket at the stern. Put a 2-axis articulated mount on the bracket and attach a tiller. Then just attach a mounting plate or clamp to the end of the tiller. You would have to work out the angles on the tiller and motor mount to make sure that the drive has the correct attitude in the water and is easily controlled from the cockpit. Starting and stopping might be problematic. You could extend the pull cord, but how do you shut it off? That throttle cable could be extended up to the tiller handle.
The advantage of doing it outboard-style is that if you don't want to use the motor you still have a perfectly good kayak. Also, if you screw up the installation, you still have a perfectly good kayak. It might take some work to modify the drive unit, but you are still left with a perfectly good kayak.
You'll need to register the yak once it is powered.
I did a quick drawing to show what i had in mind. It is a little hard to see, but hi-res is available on request.
