Build?? You mean assemble. To me, "build" means from the chip level and I doubt if anyone is doing that these days.
I built a PDP-8 work-alike from scratch a long, long time ago. I used the programmer's manual and instruction set description. I have never seen the detailed PDP-8 block or circuit diagram. But that would be impractical with today's far more complex machines.
Maybe you could compromise and "build" much of the PC from field-programmable gate array(s). A challenge in itself.