I would get a second opinion and see if you get the same results and then go from their.
If you decide to get a new motor I would highly recommend a Suzuki Four Stroke. I have over 600 hours on mine without a lick of trouble. A Marine Dealer here that sells Suzuki and other brands and works on all of them told me he services several Suzuki motors that are in rental fleets with over 6,000 hours on them and they've never had to put a wrench to them other than oil changes and maintenance.