In reference to the initial entry, not the reply...
Actually, almost all bikes produced today are 4 stroke, fuel injected and have cataletic converters, at least the "larger" bikes. (over 250cc)
The percentage of polutants that come out of the "smaller" engines is quite high for what they are, but it is still less than the average automobile in total (the total of a 50cc engine is still less than that of a 3800cc buick engine for example). The problem becomes putting more emission equipment on the 50cc engine and adding the weight would require adding more actual displacement to haul the weight and have the power necessary for even calm riding. This can become a vicious cycle.
This is one place we are definately flawed in our emission thinking. Rather than giving a percentage that the auto manufacturers must comply with...give a total and let them figure out the best way to achieve that with their engine.
If someone can make a 460ci engine that meets the standards...they would be a rich genius
Jim