Finding parts for the Tomberlin Firefox 620 can be difficult. There is a website called alphasport parts which seems to be very expensive. I was able to find most parts on Amazon by just searching Chinese GoKart or Atv. Below are the links I just used.
Brake Caliper https://www.amazon.com/gp/product/B08...

Brake Master Cylinder
https://www.amazon.com/gp/product/B00...