If all you want is the functionality, and don't care how it works, then why in the world even bother emulating, or implimenting, a z80 with an fpga?
Either use emulation software if you just want the experience, or just use normal contemporary hardware and software with no vintage aspect.
Are you actually running a z80 to run a spreadsheet for actual productivity? Of course not. So you must be using a z80 for some other reason than it's actual functionality. So, the works matter after all.
Pointful uses of an fpga wrt to vintage stuff is replicating parts that can't be purchased any more, or as a rapid development environment, emulating a system so you can develop some other hardware or software for that system etc. Emulating a cpu or a system which still exists, just to use it? Pointless. Any justification mentioning availability, size, cost, reliability, convenience etc, are even 10 or 100x better served by pure software. The amount of cases where the ultra cycle accuracy matters these days is actually quite small as both the emulator software and the contemporary cpus have gotten better while the target has not gotten any harder.
Yes, a cpu implemented on an fpga is somehow not real, It's more real than pure software, but it's still essentially pointless in most cases, because it serves neither the "I just want the experience" as efficiently as pure software, nor the "I want to run the original hardware" at all, not merely less efficiently.