These are for the most part digital chips. If you make a cycle accurate replacement, they're close enough in that they for most users won't change the output. E.g. here's an in-progress cycle accurate 6510/8500 replacement, that is if anything more compatible than some of the alternative "period" licensed 6502/6510/8500 designs, where support for the undocumented instructions vary:
There are other similar projects. This notion that the use of FPGA's means there's some inherent difference in fidelity that newly made silicon wouldn't have is nonsense for parts like this that are so far from pushing the limits of available FPGA's.
https://1nt3r.net/j-cpu/
There are other similar projects. This notion that the use of FPGA's means there's some inherent difference in fidelity that newly made silicon wouldn't have is nonsense for parts like this that are so far from pushing the limits of available FPGA's.