There is an engine out there that I think I'm safe in saying has carried almost every manufacturer's livery at some point in it's production history. - It's the square plastic case with a vertical throttle lever on the front. Nice & slim, and some (Def. Tohatsu & maybe Mariner or Suz) badged ones were 3.5HP. They have been sold as low as 1.5 Hp (presumably for tenders & canoes). Weigh about 12Kg fully fuelled, F/N gears and are slim enoguh to fit on an SR4 transom.
Other option if you don't mind going a bit older is the Johson / Evenrude twin pot 4. They are also relatively slim, weigh about 13 KG fully fuelled and have the advantage that it will still run if you loose a cylinder. Newer ones had a F/N select, otherwise it's a permanently in gear Stop / go, so need to make sure you are pointing roughly the right way when and / or have a plan before it fires.
I've got one as my Aux, and it took me the entire N/S leg of Loch etivce last summer during the 70 Wild miles event. Used it because it was better that at 3/4 throttle than the main at ticjkover oiling up at canoe speed!
I think Pol has the 2Hp single cyl version as his Aux. Plenty of them secondhand.
Other option is two of the first one I mentioned & have a 2Hp each side of the main!