You can use a lean mixture in your bike to avoid too much smoke. Just don't go too lean or it will result in engine damage. Using a lean mixture will wear out the rings a bit early, which means the machines at the manufacturer's plant will have to churn out another set for you (more pollution).
You can also use an eco-friendly 2-stroke oil.
Mythbusters did an episode that found modern motorcycles (including 4-stroke) pollute more than modern cars. Cars carry lots of technology that minotors their engines, and bikes just aren't that efficient yet.
Don't feel bad. If Al Gore feels it's OK to fly around in his jet, I think your 200cc bike is insignificant.