No they are only Legal with a Plate and Green Book which is another story with many Posts on the Subject in the Forums. You can pay the Thai Compulsory Insurance which they accept but for no reason as it is still illegal without a Plate and therefor not allowed on the Road. Depends where you are and who stops you as to Confiscating your Bike, quite a few Guys get away with it up here in the North but the Lower you go the Worse it gets and in Phuket Confiscation would be expected. Generally a Fine would get you going again but you never know???
Johnny, I don't know if you have noticed that Ohm's bike shop in Chaweng now has red plates on his unlicensd bikes, with Samui Big Bike stamped on them. Ohm has now an arrangement with the police that he now has his non plated bikes registered with the police/transport office, and the renters can now ride them legally, on Samui, without being pulled up and having to pay a fine. Ohm has a wealth of info on importing bikes etc, as all his big ones are plated, think he had about 12 Harley's last month when I visited him.