This video shows how to fit a 120mm disc brake to any Hoverboard wheel. Note that this video is only a guidance and please feels free to DIY in your way and use the resources you have.
See below links for the parts you will need it:

Disc Brake Adapter 6 bolts:
https://www.amazon.co.uk/gp/product/B...

Brake Caliper and 120mm Brake Disc:
https://www.amazon.co.uk/gp/product/B...

Music:
Your Story by MusicbyAden |   / musicbyaden  
Music promoted by https://www.chosic.com/free-music/all/
Creative Commons CC BY-SA 3.0
https://creativecommons.org/licenses/...