I've also had good luck with blue locktite on 4-stroke mufflers. I've also seen people use the orange high-temp RTV silicone with good luck. they smear it on the threads and lock nut. It just looks crummy in my opinion.

You should definitely check the valve adjustment but keep in mind that ringed 4-strokes don't always have the same compression "feel" when turned by hand as you'll find with other types of engines.