I did my '80 a while back. Here's what you will need:
A disc brake rear wheel, also the right side spacers or a way to make some.
The rear disc.
The caliper and its mounting bracket.
The brake hose.
The master cylinder.
The reservoir and its hose.
The pivot for the rear brake pedal (that's the splined piece that goes through the frame)
There is some welding. The drum brake rear brake stay rod can be used but it has to be shortened.
The stay rod bracket needs to be moved to the top of the swingarm. It can be left on the bottom and the caliper hung upside down but that makes bleeding the brakes a real chore.
The brackets for the brake hose need to be added to the swing arm.
Two 8mm ID tubes need to be welded to the frame reinforcement above the swingarm pivot point. These are the mount point for the rear master cylinder.
Tabs need to be added to the frame to mount the reservoir.
I added a brake pedal return spring so I welded a bracket to the underside of the frame. While I was at it, I added an adjustable brake pedal stop.
The pivot from a disc brake bike has the hole for the pin in a different place than the drum brake setup. If you are not afraid of cutting and drilling, the drum brake pivot can be modified to work with the master cylinder, that saves having to find a new pivot.
Is it worth it? I like disc brakes overall mostly because they self adjust, are not prone to heat fade and take less effort to stop the bike, but on the rear it's sort of a personal preference thing. That's because 70% of the stopping power comes from the front, the rear is sort of just along for the ride. I did mine mainly because it's a street tracker and disc brakes are part of the 'look'.
