The "identity" of a bike is the frame. This has the VIN. Generally all other parts can be changed without changing the identity. This is reasonable because if you need, say, to replace the muffler or the seat etc., then that does not change the identity of the bike.
It is a straightforward process, but kinda labor-intensive!
1. Buy a Thai registered bike.
2. Remove all the parts from the frame of the Thai bike.
3. Repair/repaint the Thai registered frame as necessary.
4. Rebuild the Thai bike using parts from the Oz bike.
In that process the VIN of the Thai bike is preserved. Changing the engine number is not normally a problem because engines do fail and need to be replaced, but if it is a problem then use the crankcases from the Thai bike as these have the engine number, but that is far more work, so best avoided if possible.