Back in the olden days before you could just buy a disc brake kit, we used to have to assemble our own rear discs. The source of choice was rotors and calipers from a Porsche 914. This gets you stock 4 lug bolt pattern, and an externally actuated E-brake lever.
I've got two adapter brackets and two calipers for sale for $80. All you need to do is buy some CB Rotohub rear rotors and put them together. The calipers I have are unknown. Pulling the lever makes the piston move easily. A set of brake pads and you should be good.