If you bow mount your electric, you have many higher thrust options available at 24 and 36 volt. Even can get transom mounted 36 at 105 lbs. thrust.

Edit - I looked up the Grumman Sportboat and see it is a square stern, so disregard the bow mounted comment..

It may be too big for your needs, but I have a Briggs & Stratton 5hp 4-cycle out board. It's a just Briggs lawn mower engine mounted on a drive unit. Air cooled, easy on gas, and remarkably quiet for an outboard. This is not a high speed screamer, just a nice small unit that will just barley plane a 14 foot aluminum skiff with 2 people and gear. Uses 1/2 gallon per hour at full throttle so a 3 gallon tank is ample for a days outing.

FYI - this motor weighs in at 56 lbs.

Placement of this query isn't that important, but I figured it would get a more attention in Fishing--a classic small craft rig going well with class 'glass--than in collecting.

That's a great boat for prowling ponds and rivers. With an outboard, I'd want enough to make slow upstream progress in fairly stiff river currents. But as a fishing craft, it will be better not laden with too large a motor, plus I'd want to be able to lug it easily. I think that's well under 10 hp The other thing I'd do is check for any horsepower restrictions on the waters fished and be sure to stay under that.

I acquired a Sportboat several years ago. It came with an old 1971 Johnson 6 hp motor. While the motor ran pretty well, it was more horse power than the boat needed and it was due for some maintenance. After extensive research, I bought a new four-stroke Suzuki 4 hp motor. The reviews on this motor were very good. It has a higher displacement and better corrosion resistance than the others you mentioned. I have put about 30 hours on the motor and have thus far been very pleased with it. It pushes the boat just as well as the 6 hp did and starting has not been a problem. I have had no problems with the gas but I run the 89 grade gas and treat it with the blue Sta-bil Marine. So, I can recommend this motor to you.

Thanks to all for the good advice, which is something I've come to value about the Forum.

I have a 4 hp Mariner purchased in 1985 that is still going strong. I believe they were made by Johnson.

I have a Grumman Sportboat as well! My experience has taught me that a 2 stroke, 2 cylinder, short shaft 4 or 5 hp Johnson or Evinrude is the best bet. The modern 4 stroke outboards are too heavy for solo excursions in that your bow will ride way too high without bothersome ballast forward. Additionally, if you trailer the Sportboat it is not recommended that you keep the engine on the transom. I stow my outboard in the back of my truck and put it on the boat when I launch. 2 strokes are considerably lighter and make this a much easier task.
For a while I had a modern Nissan 3.5 hp 4 stroke and it was an abomination at idle and low speed trolling. Wide open it was fine, but what good is that to a fisherman?
I am aware of the ecological benefits of a 4 stroke and I don't mean to dismiss them out of hand, but for really small engines, which is what is called for on a Sportboat, I find a 2 stroke to be too dependable, lightweight and inexpensive to both purchase and operate to resist.
I do paddle and row it a lot too.

How about 24v electric?

I too, have a Grumman Sport Boat, picked it up used in the late 70's.It came with a 10 hp, 2 stroke Mercury outboard, worked great in the big rivers (Yukon & Tanana) up here. I got a good deal at a sport show on a 2 hp, 4 stoke Honda, starts easy (like all Honda products I have had), sips gas, easy on and off, as it is very light, great for lake trolling at high idle and pushes the boat right along wide open with two fisherman and overnight camp gear. Haven't tried it on the big rivers yet or any river for that matter.
Can't say enough good things about the Grumman Sport Boat either.

Enjoy four stroke motors for environmental reasons and a noted above they are quiet. Buy fresh high grade or sport gas and you should be all set. Biggest complaint on vintage Honda four stroke is that it does not allow me to troll as slowly as I would like at times for the light canoe shown above. Heard this issue has been addressed with newer model small Honda four stroke motors, yet no first hand knowledge.

Have used two stroke motors on all manner of small craft with great success in the past. A traditional sporting camp I know of seeks out vintage two stroke motors as the owner feels they are superior to the latest offerings and four stroke motors in general.