That's not realy ture it's the toll ways mainly around Bangers. I rode in the car lanes as it was safer having a bigger bike, when I did actually get pulled I just explained by standing at the side of the street with the police for 2 minutes just watching the mopeds being dangerous then they let me go on my way.
Smile and knod alot if they give you shit, and My Kow Ji (phonomically) 'Not Understand' works wonders.
Just head generally north Pattaya
Bangers Ayutthaya Nakhon Sawan Phitsanulok Uttaradit Lampang are the major places to see before Chiang Mai, if you need a hand go to the GTRider site for more info.
And then once you feel the need to move on, theres a wealth of routes up north on the Mae Hong Son Loop.