A week or so ago I had this crazy idea that I could make a "copy" of the 6502 at the transistor level, but using a process with and open PDK.

Below is a pre-layout sim where I replicated the visual6502 extracted chip using GF180 PDK. Transistor are mapped 1:1 (depletion nmos replaced with pmos for pullups).

Test software is https://github.com/trebonian/visual6502/blob/master/testprogram.js

And you can see it fetching instruction, jumping and writing return address, then incrementing 0x10 (including double store) and then return.