You can actually do that kind of modification? To change a 250cc scooter to become a 350+cc? I have an ADV160. Can change to 250+cc?

For the X-Max and some other maxi-scooter, can.
Because the block, piston, camshaft, etc are all available as original factory components. So can just plug & play.
As you know, in Malaysia the X-Max is 250cc but in Singapore, Thailand, Vietnam, Philippines their X-Max is 300cc.
So there are many original Yamaha parts to go the big bore route with minimal cost.

I'm not sure about Adv 160. I thnk maybe have some brands (maybe Thai or Indo brands) that might have.
Correct. Legally, no. But if you see those X-Max on convoys laju laju type, majority are all big bore.