I have a 2000 2800SCR with the single 7.4 MPI. Mine too was under powered when loaded up and four people was the max. I installed a K&N air filter then tuned the engine and now can get eight people up on the plane.
I would do a compression check, tune up and see if K&N makes an air filter for your engine before looking outside. I did the same thing, looking at bigger tabs, prop, etc, when all along all it needed was a tune up.
As for hiring a lawyer, well, I am skeptical. I am a litigator who handles business disputes and I just argued a case on a failed transaction before the US court of appeals yesterday. If you didn't spend the money on a lawyer to draft a contract for the transaction, then I think spending the money afterwards is foolish. How are you going to prove your case? You did not hire someone to check the computer and I don't see you recounting that you told the seller if these engines dont have 75 hours I'm not buying. Plus what are your damages? It sounds like you had an appraiser who beat this guy down on price already and that would be your most likely remedy.
Getting the contract rescinded requires that you immediately demand it from Seller. You didnt do that and have used the boat. So, either you make good on your threat and get a lawyer right away or keep the boat.