Thanks Chris. I will take a look. From what i've read i think that the only serial number is on the engine. According to Service Canada you don't need a HIN # to register a boat that was manufactured before 1981.
I would think there should still be a capacity plate mounted somewhere. It might be on the transom, or it might be inside. That plate might also help you as it might have a HIN or serial number too.
Essentially back in the mid 70's it became the standard for all manufacturers to start stamping HIN's into the rear right of the transom area. I can't remember the year it became law. But basically how it works now, is that the first three letters are a manufacturer's identification code and then the last two numbers are the year the hull was manufactured.
The site I linked to below explains it pretty thoroughly.
Just keep in mind that there might have been slight differences from country to country in and around the mid 70's and then later things changed in the mid 80's as well - although things seem to be more standardized now from what I've seen.
http://boatsafe.com/nauticalknowhow/hin.htm
-Chris