I'm wondering what are good price/performance modifications for this motor
(1997, in an inboard ski boat). Specifically, what modifications that would
commonly find on street of off road vehicles, such as heads, cam, MPFI kit,
intake manifold, supercharger, etc, would not work or not work well on a
marine engine application? Obviously exhaust is one of them.

Secondly, how will the existing fuel system cope with additional horsepower,
or is an aftermarket open loop MPFI system one of the required
modifications?

The boat has enough low end for my purposes, so I'd like to bump the power
range to 2000-6000rpm and get another 5-10MPH (70-100 hp?) out of the boat.
(Let's assume it is or will be prop'ed appropriately)

My background is in street and off-road, so any marine application advice is
appreciated.

Thanks in advance.