Cars and boats may share the same base engine, but are completely different entities all together when it comes to fooling around with them for extra power. You usually can't get away with doing the same things to hotrod a marine engine than you do to an automotive engine, at least not at a reasonable cost.
Quite honestly the cheapest, safest, and best solution to getting more power out of your boat is finding another boat with more power. It sounds heavy handed, and probably something you didn't want to hear.
When you start talking about adding automotive parts to your boat to make it faster, you really need stop and think why rational people aren't normally doing this.
Here's a couple for you to consider;
Safety
Certain fuel, ignition, and electrical parts are marine rated for a reason. It's NOT so they can charge you more money for the part. It's because if that part fails, its less likely to cause a fire or explosion. Dozens, if not hundreds of people find this out the HARD way every year. If they're lucky, all that gets damaged is the boat and their pride.
Reliability
Marine engines work in a very hard environment. Unlike a car, they are under constant load. Fuel and air mixtures are critical to keep the engine from detonating, which will blow holes in the pistons. Cobbling together a bunch of parts from various sources with out any thought or knowledge of their relationship is a recipe for a very short lived marine engine (or any engine for that matter). When that engine fails, where are you going to be? There's a 99.99% chance you wouldn't be any place where you could get out and walk. All you can hope for is that you're not on fire or sinking, and you have cell service or radio contact.
Just my .02?
