Yoga,

Not sure about the plug threads. Are you using the washer that comes with the plug? You might try a NGK CM6... I didn't use the plug that comes with the DL.

The coil spring goes in the spark plug cap. I'm guessing it's a spare. Your spark plug cap should have the spring installed.

The pitts muffler will cut the rpms down some.. but these engines are very strong... I doubt you will notice the slight drop.