Not easy. A lot of trouble & for 99% of people not worth the time, trouble or cost. So buy local & save yourself heaps of time, worry & probably money. However if you insist, and think you can out smart the system, then consider these facts.
In Chiang Mai Khun Nat at Piston Shop is helpful sorting out bike registration.
Still unsure? Then read some of these GT Rider Bike Import experiences and comments
The GT Rider advice: buy new locally & get a legal registered bike & registration book, with no fear of having a dodgy book & getting your beloved bike seized.
New bike dealers in Thailand are