It is not crucial that these parts are exact but you will save yourself a bunch of work if they are close.  Once they are all glued together, you can sand the inner radius to make it perfectly smooth.  This will be much easier if the inner radius of all the parts are close to begin with.  The outer edge of the spacers is less important because it will be hidden under the lip.  The lip itself can be any width you choose as long as it is wide enough to hold a skirt and/or cockpit cover.

If you do decide to build a second boat, please respect CLC's property rights and buy a license for it.

That is a very interesting concept.  There's a bunch of truth in "build your first boat second".  I made templates from all of the non-scarfed parts for both boats (Eastport Pram & Passagemaker).  I may have to build another EP for my son (and yes will pay CLC if/when I do so).

It's an interesting concept to build two boats at the same time.  I estimate that it would save about 30% of your time overall, since setting up and cleaning up time would be divided by 2.  I was also able to cut out the templates at the same time as the boat parts, so you could cut out two boats and a template at the same time pretty easily.  It's also cool to have two of the exact boats for match racing, or just giving everyone a chance to mess about at the same time instead of taking them out in shifts.

With regards to precision, for some parts, I rough cut with jig saw and Shinto rasped to the line, for others I used a Japanese pull saw on the line.  My plans cut out parts fit like they were CNCd, so I would get as accurate as possible in order to avoid any unpleasantness later on in the process.  When stitching up your parts, you want to be able to blame the stitching, not the parts.