I'm in the process of licensing my KDX. I have done a little research on this topic in Missouri and found that the following are required:
A rear view mirror
an electric horn (was told it must be electric)
Front and rear lights (Not a problem with the KDX)
Insurance (which means you will have to start paying Personal Property Tax)
Keep in mind the mirror and horn are only required for inspection. (These items may be removed once you have passed insp.)
Also, It use to be required in the early 80's to early 90's that your tires couldn't say "not for highway use" and you had to have a forestry approved spark arrestor if you wanted to ride on National forest land. I know the spark arrestor still applies, but not sure if the inspectors will check your tires.
I was interested in doing this to ride in the Mark Twain National forest (Sutton's Bluff area) and around Potosi.
Just my .02 cents
dave