The van de Stadt web site offers sets of plans for many of the older designs. I would try them first. Even if you aren't a Dutch speaker I would try to get them on the phone instead of Email. (The website address should be listed here if you do a search).
This boat is still popular in the UK, but I've never found any place on the internet where owners congregate. I think the man who built many of them is still around, but I don't know how to contact him. Do you know the name of the original builder?
If I come across anything more helpful I'll let you know, but that's the best I can do for now. I'd be interested in hearing how you make out.