Briggs and Stratton lost power

I have a four year old Grasshopper 226v zero turn mower with a commercial v twin Briggs and Stratton engine. It has always run and cut great. This year a belt pulley that attaches to the transmission came off. I tried to reattach it but it kept spitting out screws immediately after starting. I took it to my local dealer and they replaced the pulley and did a routine service. When I got it back it mowed great for an hour. I stopped to move some debris. When I started it back up it sound like it was running on one cylinder. I called my dealer and dropped it off. They had it for two weeks saying they thought it a fly wheel issue and had to call a Briggs tech. They brought it back and said it was fixed, and no charge. Well it ran better but still backfired, would only run with the choke out full, and had limited power. I took it to a Briggs and Stratton authorized repair shop. They pulled and cleaned the carb, replaced the fuel pump, and said someone had adjusted the exhaust valves wrong. The said it was running much better but is underpowered and I should replace the engine with a Kawasaki (I’m considering that idea). Well now power wise it runs about 75-80% of what it originally had. It runs with the choke in and is smooth, but it’s like it’s lost a gear. I still smell it’s running rich and there’s a metallic sound when running, almost like the pinging of valves in a car with bad gas. I have run sea foam through it twice and switched gas brands. Any ideas of what happened and how to fix it?

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