Everything I have read indicates the drum (when working properly) is all you need. If you can lock up the rear tire then you can't get it to "grab" any more! A lot of people complain about the rear disc brake claiming that it does not brake as well as the drum but I can lock up the rear wheel on all of mine, disc or drum.
I suppose it would be possible to fit the rear disc components from an early R100RT or R100RS but I've not done it. You would need the rear wheel, caliper bracket, rear axle, caliper, lines, rear master cylinder w/linkages, brake fluid reservoir, etc.