Okal Haus still exists. In the area around Salzhemmendorf, there might also be several carpenters who were employed or trained there during the old Okal Haus days. Around Paderborn, these houses are not uncommon either, and the former Okal Haus style is, to put it mildly, not hard to recognize. In this respect, you can tell from the houses whether it’s worth asking the owners there. Take a look around housing developments from this period. The internet is a fine thing, but with open eyes in real life, you can see quite a lot as well.
To summarize: my first point of contact would be the manufacturer itself; followed by companies where insiders from the former production can be found; and lastly, people in the same situation. For a construction year of 1980, you won’t be the first to want to refurbish the place.