You won't be able to make anything remotely close to a modern sync sound camera like an Arriflex or Aaton (which took teams of experts, decades of experience and cost tens of thousands of dollars per camera), but maybe something like a Bolex or the like is possible, with a few years of effort. You'd need to buy the optics, and spend time researching viewfinder design. The closest thing to camera schematics you'll probably find are service manuals, but they don't typically include exact dimensions and materials. Bolex had pretty good manuals if you can source them. Otherwise I would suggest taking a camera apart to study and measure the components. Make sure to run down the spring all the way if you take apart a spring motored camera!
I documented a few camera disassemblies here:
Making your own camera certainly won't be a cheaper option, if that's your thinking. But if it's a challenge you're after, go for it!