Official all modifications that you do to a motor-vehicle are illegal in Thailand. It is up-to the police officer on how he interpret the law.
If you want to modify a motorcycle, and want to do it legally. You need to modify it, and re-register it at the Department of Land Transportation. Depending on the sort of modifications this go's easy or difficult, to some degree it also depends on who helps you at the Department of Land Transportation.
in the phuket gazette online, there are often mentions of people getting tickets for "illegal modifications" to motorbikes in the police activity section. what exactly are "illegal modifications"? up here in in the village outside lom sak, the kids modify the heck out of their motorbikes, remove the body work, add chrome and noisy tail pipes etc. my wife tells me they couldn't go into town on these bikes cause the police will stop them. you can't remove the body work???
what can you do and not do with a motorbike? naturally i want to build a chopper, extended front forks and the works. but doesn't sound like i can ride it anywhere outside the village.
any links to these laws in english? thanks