I have tried to quite down B&S engines and its a loosing battle. I even made a water muffler. The engine itself makes 80% of the noise, not the exhaust. Also never messed with a 318 but had a 350 out of a boat. I hooked it up to a 1 gallon fuel tank to test run it. At 900 rpm it used the gallon of fuel in about 30 minutes.
I have a 8550 Troy built gen with a B&S engine and it will run with my home load(fridge, freezer, furnace blower, well pump, tv, lights, micro, stove, ect) on 9 gallons a day... I have been without power once for 8 days straight and once for 3 days plus plenty of 1 day stints.
Get a pto genny for your tractor if you need more power and quiet. HF has a 12K unit 15K for $1000 or just live with the noise. You could sell the 318 on Ebay or Craigs List and buy a new genny.
Chris