On spark plugs, you should be using whatever the manufacturer recomends, or an exact replacement from another manufacturer. Under some circumstances, going up or down one heat range is ok but the reach, or thread length, should be the same as the original. Using a differnt reach can cause the plug to hit the piston, if longer, or allow the threads to gum up and lower compression if shorter.
The single speed problem is more or less insurmountable. When you take off from a stop, the clutch wastes power generating heat until the ped is going fast enough to match engine rpm. You might want to play with the throttle when engine rpm is low. Most engines do better with a smaller throttle opening at low rpm.
Yuo are right about the bigger carb/exhaust when increasing displacement. If you decide to try this, look for another Yamaha with larger displacement and compatible parts. If this does not exist, I would stick to a slight increase in compression and learn to smile at the irate motorists.