We have a couple of old 2-strokes that I've used side-mounted on canoes.
One is a 3 hp Johnson from the mid 1950s. Two cylinder, water cooled, integral gas tank. Not real light, but OK on big canoe, plenty of power and smooth. Worked pretty well side mounted on a 17 ft Grumman, would be even better on a square ended canoe.
The other is a little air cooled Sears 1.5 hp. Don't know the age, bought it used from an uncle 15 or 20 yrs ago, could be from the '60s or '70s. Single cylinder, integral tank, noisy, but a lot lighter than the 3 hp motor. The lack of a water jacket for cooling definitely lets more noise thru. Mainly use it on a 16 ft Old Town Discovery canoe with side mount. The much lighter motor feels safer, tips canoe much less, still pushes it along pretty well. Don't know if anyone makes little outboards like it anymore.
If you get a 15 ft or larger square back it could probably take up to around 5 hp (but check the actual rating!) If you can find a used 3 to 4 hp in decent shape or worth rebuilding it could save you a lot of money. Your're not going to get a lot of speed anyway, canoes don't "plane" or get up on step with a motor. Too much motor just hangs excess weight on the back and imbalances the boat. I haven't looked at new outboards lately, but I think that for the same power 2-strokes are still lighter than 4-strokes.
