I don't know of anyone who sells rear disc-brakes for puch. It has been done tho.
You will need to lose the stock swingarm and get something beefy, so you can weld a caliper mount on it. Then find a hub or wheel that has a sprocket and a discbrake or make a disc-hub yourself. And then get it all together :P
It isn't that hard actually, just a lot of work.
I'll find you some pics later.