Check the input voltage on the original power adaptor. Does it say it takes input from 110 to 240 Volts? If so, it should work. You just need a plug converter to make it compatible.

The power supply shipped with the Mi Box outputs 5.2v 2.1a.

It is unlikely you have a spare matching this level.

Make do with the adapter unless you want to pay for an EXACT replacement

Assuming the US adapter is rated for 240v, I'd stick with the provided power adapter and find an adapter that clamps on to the plug. Mine has an EU plug and I use this adapter, which works great - https://www.amazon.co.uk/dp/B01187AJH2/ref=cm_sw_r_cp_api_EdfXybNBCKMS7 .

5.2v is quite hard to find and some of the early threads on here talk about under-voltage killing the Mi Box. Not sure how true they are but not really worth changing it considering the provided adapter is good quality.

just make sure its 5v 2000mA.
i was able to use other power adaptors from other android tv boxes. you can also look for a genuine psp power brick. the model number should be PSP-100. They have 5v 2000mA. I'm not that sure if the quality of those universal power adapters with adjustable voltages are good.

I really don't recommend using these off the shelf multi voltage power adapter. Their output voltage are usually not clean and usually under spec. It might do more harm to your mibox using these Universal Power adaptor.