that hole you see is for mounting a fender... not sure if i'd trust that for a brake.. some fork bridge designs might be beefy enough.

you'd need a BMX rear caliper brake to fit that thickness... most larger front calipers have a longer bolt to mount them in the crown or the lower section of a unicrown fork's steerer tube

the Tektro R559 calipers are well loved... but are SHORT pull, side pull type brakes, so get the correct lever/s to match, or you WON'T like the results..

check the rims on your bike... they may not have actual Brake Tracks, as mentioned elsewhere!

i'd just find some Avid BB5 or BB7 mechanical cable disc calipers.
be aware that they come in two pull ratios... long/MTB pull, and short/Road pull.



and frankly, hyd. hose fails are really rare. was it routed inside of the fork leg, as it should have been?
if the failure was higher in the run, then ANY hose or cable would have gotten messed up.