Hey bigray, Welcome to the forum!
Is this a new motor/ Did it always knock or is this a new development? Does it seem to run O.K.? Could be many reasons for a knocking sound.
Did you check to see if Head bolts are loose? What I mean is don't start tightening head bolts nilly willy Start with a cold engine - just each one, check to see if there is give. If one gives a little, move on to the next in an X pattern and take what gives and call it good. We're checking for looseness not tightness. I hope you know what I mean. Muffler same thing - If a little loose take what gives. Do not over tighten. Give a little history on the type 2 stroke? 4 stroke?