Weaver's "Desperate Measures" is probably the best single source of information about the German Sten type firearms.
For dimensions, a bit of reverse engineering is going to be necessary, for the front end - barrel block, barrel, front sight, magazine housing assembly, bolt. Fortunately, for a fairly accurate reproduction, the good news is that the originals show quite a bit of variation.
Home made firearms are nothing new. I registered my first pistols about 40 years ago. Over the years I have seen just about every type of home made imaginable, from muzzleloaders through legal submachine guns (in the good old days) through precision target rifles. There are websites dedicated to legal home builds. Not talking about "building" something like an AR from a bag of factory finished parts, of course, but fabricating major components from scratch.
Just yesterday, I bought a registered home made .22 auto pistol. Haven't had a chance to study it in detail yet, but it is really interesting. It is not a copy of anything I have ever seen, seems to be a unique one-off.