Hi. Good luck with your Sea Couch. I used to own a PWC rental business and bought a new 1991 Sport Cruiser. I still have it. I have young kids, and the SC is great for riding kids around because of the side-by-side seating. You can talk to them without taking your eyes off the water.
After I closed the rental business, I went totally through my SC because it was thrashed. Rebuilt the engine, re-did the pump, painted it, and re-did the totally ruined seat. So, with this experience I offer an educated guess as to your issues.
1 Glad your replacement of the plugs made a difference. To check overall condition of the top end, take a compression check with a compression gauge. With a stock head 100-150 PSI is a ballpark of what you’re looking for.
2 Make sure there’s cooling water coming out the little pisser on the side of the ski. This tells you water is successfully circulating. I had some clowns run my SC onto a sandbar and pump sand through the cooling system, plugging it. They then continued to ride, seizing the engine.
3 I had a continual problem with people sucking up debris into the pump. The intake grate is hard to check because you have to go almost or fully underwater to check. Thus, the renters didn’t do it. Sucked up rocks as well as the previously mentioned sand. Destroyed the vanes in the pump as well as the stock POS impeller. (The SC is a big, heavy machine. The stock aluminum impeller is woefully inadequate for the job. Will get cavitation burns like crazy, even if you careful about checking for debris. A stainless aftermarket impeller will make a huge difference. Make sure you get one specifically designed for the SC.)
Cosmetically, the pink is out of control! Too much pink! I removed all the stock decals and repainted it the same white color. I found a seat from a newer SC. The aqua one. The newer aqua blue machines had a gray seat base instead of the pink. I used it and had a nice, leather pink seat cover put on by a marine upholstery shop. It has lasted 12 years and looks new! The removal of the pink decals and pink seat base really toned down the impact on the eyes, and mine looks great. Just used it a month ago and it ran top notch.
One other thing I did was remove the oil injection and go pre-mix. Remember, there’s no idiot light that goes on if the oil pump quits working. Your engine simply seizes.
I have a nice 750 Kaw engine that would really perk up the power of the SC, but I'm not going to do it. After all, this is not a race boat. Never will be. The stainless loop on the back of the machine looks like it's there to tow things, but is not reinforced in any way. I had a renter try to tow a person on an inner tube and tore the loop right out of the transom! If you reinforced the area and replaced the loop I could see how more power could then be utilized for towing people. But, mine works great just for a sight seeing machine that can go maybe 35MPH. No need for more speed to do that. Plus, aside from the oil pump removal, is totally stock, so runs perfect, starts every time, and is easy on fuel. I rode my Limited spec Superjet on the same vacation a month ago. Rode the Superjet half as much as the SC and burned twice the fuel!