LEGO offers schematics for the Ev3 brick in their hardware developer kit accessible from the bottom of the Ev3 downloads page. (Files 1 and 2 in the zip) Do these schematics contain enough information to theoretically build an Ev3 from scratch?
Based on 3 years of extensive study of the internal workings of the EV3 as part of the ev3dev project, I can confidently say that if you build that schematic, you will get something that very closely resembles a mass-produced EV3.
There are some minor differences though. The HW Version Identification shown on page 3 of the schematic (0101 [binary] == 5 [decimal]) does not match the actual mass-produced hardware (1000 [binary] == 8 [decimal]), so it leads me to believe that the schematic that LEGO has shared is not the final revision, but rather a slightly older revision.
There is also a PIC microcontroller (U28) and MFi authentication chip (U29) on the actual EV3 that is not on the schematic. The firmware source code seems to indicate that this is used for the iPhone/iPad/iPod Bluetooth mode somehow. So, this feature will be missing from your clone EV3.
I have also just noticed that the "3 SEC delay" circuit on page 3 is not populated on the actual EV3. This is further evidence that the publicly available schematic is not the final revision.
One challenge you will face in building your own EV3 is that there is no PCB layout provided by LEGO, so you will have to do this yourself.
You will also have to find a replacement display. The LCD does not appear to be generally available. (I broke one and looked for a replacement several years ago. I ended up replacing it with a color TFT instead.)
Also, while we are on the subject, there is a BeagleBone cape called the FatcatLab EVB. There is an open source schematic for this as well. If you are really going to build your own EV3-like circuit board, it may be more appealing to copy this one because it has a faster processor and more memory.