I know what you mean, mine is a Sears boat from the 60's or something. It has a model number but I couldn't really find anything on the Internet about it. I couldn't find a VIN on mine either.
I have a similar boat and was told when I was researching getting it registered a couple years ago(I still haven't registered it), that you don't have to have a title or anything. A bill of sale would be enough documentation, if anyone even asked for it, when you register it. The title is required if the boat is bigger than a certain size but I don't remember what that size was. Good luck selling it.