Thai law doesn't allow any modifications to any vehicle once it has left the factory ...
However if you can get a Thai doctor to give you a disabled certificate stating you can only drive a 3 wheel scooter, then you can get your vehicle legally modified and can legally drive it on the roads, and when it's 5 years old, accompanied by the doctor's certificate, it can pass a DOT test and so can be then taxed ...
Hope this helps ....