If it is a smaller hub and not a crazy fast bike you could do something like this. Drill through the hub and use some stand-offs. Weld a tab to the lower leg and bam easy disk brakes
I used a setup like this on my hobbit. With the cable brakes it stops pretty well. Somewhere between really good drum breaks and pit bike style hydros like Joe posted. People say your going to warp that rotor easy….your not going to stop well…..the pads will wear out easy….bla bla bla. Don’t knock it until you tried it. It has lasted for years and I just now should probably change the pads…I mean Jake uses MTN bike hydros on Toby?!? Fastest bike I have ridden and stops amazing.
I will say that the link that Joe posed…That setup is way superior to MTN cable brakes. My bike with that style of brakes will toss you over the handle bars if your not careful…..but hey MTN brakes is just another option.