The difference is easily explained.

A jet boat survives on a jet of water pushed out the back of a hull.
A prop boat is pushed by a propeller at a much lower and positive point. 

My last I/O boat had a small block in it and ran in excess of 65mph, with four people,  a dog and a cooler full of beer and soda. If you are looking to have that same performance in a jet, you will need to make well over 600hp and have the set up totally dialed in. Not to mention burn nearly three times the fuel.

One way you need to look at this that is somewhat eye opening.

On a hull such as yours, it would take a huge gain in horsepower to  make even a modest difference in performance.

To put it in perspective,  a peanut port headed 454 makes very close to 330hp in stock form.

Heads alone might be good for another 30hp. (Even that is a stretch) to make the heads work, you would need a cam change, bigger carburetor, different manifold, potentially larger fuel pump, different exhaust,   just to see the potential of the heads. This leaves the compression ratio stock (probably 8.8:1) with likely cast pistons and crank. You will barely notice a 100 hp gain (maybe another 5mph), which you still will not have with the above mentioned mods.

So in short, spend a bunch of money to break it. Woohoo...

I would leave her the way she is while scrounging parts and pieces to build a killer (if that is what you really want). This way you get to enjoy the boat while you build a different motor.

Good hunting.

GT

2001 corvette starter 168 tooth flywheel. Google it. Autozone. Works great on mine. Lifetime warranty. Lightweight.

And finally, today I learned that the radio shack by my girlfriend's house is going out off business. And as such everything is 80-90 percent off

I ended up with all of these solderless connectors for a little less than 20.00