Yes the 66cc is still powerful enough to get you around. The biggest difference in my opinion between the 48cc and 66 cc is the 66 has a little more torque.
Yes, you can use a different size rear sprocket for more torque. Most kits will come with a 44t rear sprocket. Thats what I have been using and I am very happy with it.
NO!! They do not pollute as bad as everybody says they do. They actually run clean enough to pass Arizona Emmision testing
AZ law requires 48cc or smaller and a 20 mph speed limit. I have the so called "80cc" but I used a electric pencil and scribed "48cc" on my engine. As long as you tell the officer that it is a 48cc you should be okay. Without them going to all kinds of trouble to measure your motor, thier is no way for them to tell.
Here is a link to Arizona HB 2796...A lot of folks will make a copy of HB2796 and carry it with them when they ride. Thier are a few cops who do not know the law.