1st off - is this a typical Reverse Rotation Correct Craft -
(You updated this in while I was typing reply ...)
Will need to know more about what you want - besides 'more performance without effecting the fuel consumption'
I build lots of marine application engines, and they are usually easy to spec. if the customer has a clear vision of need(s).
That said, I've found most have responded very well to a swap from Carb to EFI - as this gets you some increased performance (along with supporting mods) and the best fuel consumption rate you can get with most combinations in the widest operational range.
There are numerous aftermarket suppliers that can furnish 'turn-key' stuff or you can piece the system together with readily available parts from auction sites and other sources, depending on you needs/talents.
The 781 heads are a good upgrade, but will need other intake system mods to shine.
The 96 and later Vortecs can also be a good mild marine performance upgrade head. Their smaller 105cc will raise CR vs. the standard 'Open' marine heads.
But to increase the cam spec.s for either (or just about any) head choice means you're into valve spring changes.
Is your cam stock?
And what is the Firing Order of the engine?
Looking forward to feedback and further discussion