hotter plug.........7C ou 6F for .21 onroad........

and 5 for .12 onroad or .21offroad........

regarding the head clearance, I wouldn't change, anyway theoretically you should increase head clearance due to the higher content of oxygen in the air (lower the temp = higher the density of the air, which means you have a bigger volume of oxygen for a given volume of air compared to higher temps)...

you just need to run the engine rich, and this is why you need a hotter plug, to achieve a complete combustion...........