It's possible to ride a 50cc scooter legally in Thailand, the only problem is that no local manufacturer sells them. Buying one outside Thailand and trying to get it street legal costs likely more than buying one of the cheaper locally available 110 ~ 115cc scooters.
The 50cc scooters which you can often see in neighborhoods in Bangkok and other cities are often equipped with a two-strokes engine. Thai environmental laws are pretty tough for 50cc scooters/motorcycles, and I'm sure that even several four-stroke 50cc engines from Japanese manufacturers will pass the Thai environmental/exhaust emission testing.
I have attached a document from the Thai Industrial Standard (TIS) to show the required emission levels.
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50cc scooter legal in Thailand?
