You would probably just get a 4hp to plain a small inflatable with just you on board, but you might struggle fighting a strong tide . A slightly larger boat like a 310 will actually plain more easily ive heard. There are some decent 5hp two strokes on ebay at minute,
Yamaha 5 Hp outboard 2 stroke short shaft boat engine (model type= 5CMHS) | eBay
that is the same as my engine , awesomely light and good size integral fuel tank which a lot of the 4 strokes don't have. Or you could get a Tohatsu 9.8 weighing only 28kg which will fly even with 3 or 4 on board but they are hard to come by and expensive.
Yamaha 5 Hp outboard 2 stroke short shaft boat engine (model type= 5CMHS) | eBay
that is the same as my engine , awesomely light and good size integral fuel tank which a lot of the 4 strokes don't have. Or you could get a Tohatsu 9.8 weighing only 28kg which will fly even with 3 or 4 on board but they are hard to come by and expensive.