I used the cable disc brake and 160mm rotor from the donor Fito bike.What size disc and mount are you using KCvale?
I am trying to use a Avid Elixir 5 hydraulic brake on the back but the mount angle is wrong and the 160mm disc is too small.
I had issues making it work with the internal 3-speed disc brake hub as well.
Since I had to widen the back the caliper mount was off so I bent it to be in-line with the disc.
After that it is just mounting your caliper where it needs to go.
Post some pics of your rear hub and I'll see if I can help but it's really just a disc matter not a frame matter, where the wheel is determines where the caliper has to go which needs a design improvement with the horizontal open back dropouts, but disc brakes have all kinds of different 'in-between' spacer pieces to place the caliper correctly over the disc with adjustments, almost like a Lego set.