None of the modern 4 strokes will fit as originally intended, unless you go with something smaller like a 6 hp.
You can modify the opening... but DO NOT cut down...
#1 the swept back design of the rudder will interfere with the motor's shaft, and #2, water is more likely to slosh into the motor well. On my boat the long shaft Tohatsu 9.9 cannot sit perfectly vertical for that reason. Which will make the boat squat in certain conditions... allowing water into the well. For long motoring I have moved ballast forward to help with that. Some owners have purchased an after market rudder whose design is not so swept back.
If you cut the opening at the top, the hatch will not close properly when the motor is tilted up... but this is the most common modification... and the one I will make when I finally upgrade to a 4 stroke 9.9 or 15 hp Tohatsu. I've seen a number of solutions, but I've decided that splitting the hatch, a long side with a hump for cowling clearance, and an easier opening short side to access the interior of the motor well.. will be the direction I go. Also... with the removal of transom material up to the rub rail, I will install a horizontal tang that ties to the backstay's chainplate which will spread the load across the width of the stern.
Finally, If you go with an external bracket, I would opt for one that lifted vertically because you'd have to extend it out quite a ways from the transom for the motor to clear when it is tilted up. This leverages an already heavy motor to even more unwieldly weight on the stern. With the vertically rise bracket and a remote controlled motor with no tiller you can tuck it right up against the stern.... like an outdrive.... especially if you fill in the now unnecessary transom opening. Best of both worlds.... heck... maybe that's what I'll do after all. You could motorize the lift mechanism and make it all remote.... perfect for this old guy.
P.S Pay attention to total motor weight...
