There's one for sale around the corner from me, I'm trying to find out about the boat too for a friend who's considering buying it. I'll share what little I know. When I Googled "Barnegat-17", your post was the first entry, so that gives you some idea of how little info is out there!
It's a small daysailer / racer, designed and built by Siddons and Sindle in the mid 60s. My drive-by tells me it's a small, mostly open centerboarder, looks a lot like a smaller Flying Scot or Thistle. I'm presuming you know how to sail and something about boats, if not just get some books on dinghy sailing, there are a zillion designs similar to this that all sail similarly-- Snipe, Lightning, Flying Scot. It sounds like only a small number of these boats were built, and a long time ago at that, so finding stock replacement parts is unlikely, but it will be easy to find standard sail hardware that will work fine.
I did see that Howard V Siddons himself recently joined the Island Heights Yacht Club, you might be able to get something from him personally.
BTW, Barnegat sneakboxes are something altogether different, small row / power boats for duck hunting. Lots of boats have appeared around the Barnegat Bay, a hotbed for all sorts of boating activity for centuries, and numerous styles of boats, as well as specific designs, wound up with the word "Barnegat" in them. The Barnegat 17 is one specific design of boat, each one has identical hulls from the same mold and the same rig.
Let us know what you find out, would be interested.