I have done it twice both for the "old" brake system before mid-1960 with Al brake shoes and with the "new" system with steel shoes. I have used Mercedes rear discs from MB W123 model 1976 -85 and Volvo 240 calipers, also rear parts model 1975 - 1993. Then you have to make an adapter plate and do some machining on the disk and hub. Some pics. here
http://www.garaget.org/?car=238954 Best Borgward regards from Sweden Lars K