There is a 40 pound weight limit for the motor mount, so a new motor would restrict you to a 2.5 horsepower. The Suzuki is the lightest at 29 pounds. Honda and Yamaha are a few pounds heavier. A Tohatsu 2.5 is 43 pounds, a bit too heavy. The 3.5 Tohatsu weighs the same as the 2.5. All these motors are 4 stroke engines. If you wanted to risk the 43 pound weight and didn't,t mind paying more, the 3.5 would make more sense in the Tohatsu, since it weighs the same as the 2.5. I believe you could reinforce the motor mount with plywood between the aluminum tubes.
I power mine with a 1978 2 hp Johnson motor which works well for my purposes. It is a 2 stroke engine, that weighs about 25 pounds.