I want to put an electric outboard on my Blade F16 when it is done. Why would you want a petrol engine? Silent, efficient, clean and low maintenance electric outboards seems like a much better option unless you are going long distances.

Depending on what you want, I think a 2.5HP should be enough to get respectable speed out of your boat under motor. The 5HP I had on my lapstrake 16 foot launch when growing up was a good choice and a FX1 is far easier driven and much lighter.