This engine is in a fairly high state of tune as it is, being a lightweight single cylinder unit. (unlike the twin cylinder 8-10hp engines that are often detuned 15hp ones)
I'm not sure you could get much more out of it. If you are going for 4m boat then you'd probably be better off selling it and buying a bigger engine.
Depends on how fast you want it to go! I'm sure a 4m boat will plane with a 10hp on it, certainly with one person on board and maybe two.