How about a Tohatsu 9.8-4 strokes 210 CC engine, it's the top of the detuned 8 HP version. Personally if counting with an air deck sib as in your case will be happy to count with the extra engine punch compared to a 6-8 HP engine, bear in mind that air deck sibs tends to deflate during boating hours which will inevitabily create more hull drag which will need to be compensated somehow..
It's a pity that 4 strokes engines weights as much as 10 + kilos compared to same 2 strokes versions which are lighter to place and retrieve from transom... A Merc 340 is rated for a 15 HP engine, anything below 9.8 will surely give poor combo water performance or strugle to plane if medium or heavily loaded.
Happy Boating