You are doing a great job. I would guess one of the most difficult parts of the project is locating suitable material....
Again, these are guesses leaning to the heavy side.:)
I never weighed it, but I would guess about 125#. It was 1/4" plywood. I would post a pic, but it is not on the net.
A good reference for you would be the Vintage Hydroplane website. Also Phil Kunz' site.
I built a 10' racing pram about 15 yr ago, it planed at about 12mph with a 5hp air cooled Sears engine.