there are generally many possibilties- ??
but that depends on the exact engine?
as example i modded my johnson 90 outboard years ago..
the 125hp heads etc fit on, with very minimal work..
if i already had a 125 ,, it could be modified,, but not with easy bolt on stuff like the 90..
but almost all the common outboard engines have a lot of potential in performance, as they are built with longevity/reliablity in mind and not shear hp.
just like the typical 2 stroke dirt bike engine.
so there is often a nice balance , IN BETWEEN, you can find if looking for more performance..
my boat isnt fast really, but does about 58 to 60 mph.. (GPS'd) as the speedos are always off..
which is fast enough on the water..lol
freshly wax the entire hull , and it gains about 3 mph.
;)