I would not tamper with the carburettor or muffler system to start with. This engine is a matched system. Once you change that, it might be better to use a standard WA or WT carb so you can adjust idle and High needles. Or you might contact Walbro for spare fuel jets of different size. These jets are precision drilled. Once you start drilling them out yourself, you know where you came from, but loose track of where you go.

Stick with the heat range of the plug that is fitted. If it is a "7", that should be OK.