Just as a point to chew over, I would say there's one reason why vintage Macs are probably a less prime target for an FPGA-based upgrade or replica than, say, the Amiga: the Macintosh was never a very "hardware dependent" platform. From the very beginning the Mac was structured around abstracting the underlying hardware through the System ROM, and writing software "to the bare metal" was actively discouraged. (And on the flip side, Macs have never incorporated much in the way of proprietary acceleration hardware into their designs; they in fact tend to be pretty basic, with plain video frame buffers, no blitters or DMA, etc.) The Amiga is the polar opposite of that, or at least the "classic" Amiga that all the old games are written for; most of the power of the machine was in fact built into the chipset. It's pretty difficult and computationally expensive to emulate all that chipset goo and keep everything it does "cycle-accurate" at the same time you're doing the CPU emulation, which is why in fact the most popular Amiga emulator, "UAE", was named with an acronym that originally translated to "Unusable Amiga Emulator".
This dependency on replicating the exact cycle-accurate behavior of the "whole computer" at the same time trying to wedge a faster CPU in there is why crazy CPU upgrades and FPGAs are so attractive to Amiga-ites. (Well, there's a bunch of contributing factors to that crazy, like of course the fact their platform died before it ever officially moved to some other CPU, etc.) But with a Mac running old software on an emulator is already a solved problem, Apple did it in 1994, officially. There's very little Mac software that really depends on cycle-accuracy, so the main selling point of an FPGA is kind of moot.
This issue of cycle-accuracy is actually the source of some really amusing running battles on Amiga forums. Do a little googling and you'll find that there's basically a religious war between the various factions as to how much you can "replace" in an Amiga or how cycle-inaccurate you can be before you've somehow "ruined it". All the true believers of course can't stand pure emulation, at least if it's on x86 computer. (The various PowerPC "Amigas" of course do just as much emulation when running older games, literally running an embedded version of UAE, but the fans of those machines think it's somehow different.) Things like the PiStorm are particularly divisive because while you do get to keep the original Amiga chipset the CPU emulation generally isn't completely cycle-accurate; you'll find YouTube videos demonstrating the PiStorm and showing how although it can benchmark in 50+mhz 68030 territory there are times because of its cycle-inaccuracy it runs games slower than the original 7.16mhz 68000 does... or if not slower, nonetheless runs them "incorrectly". The Vampire itself has its critics because, again, despite being ridiculously fast it's also not necessarily completely 100% accurate when it "needs" to be... etc.
Ultimately I guess when things get completely ridiculous, like people making FPGA replicas of the 6502 that can make an Apple II run at the equivalent of the better part of a gigahertz, I start wondering what the point is other than shock value. (Obviously none of the original software base is really "improved" by this, in fact most of it breaks.) If you could make an FPGA accelerator for 68040 Macs that does as well as the Vampire does (IE, it claims to be about 10 times faster than the fastest real 68040) maybe that's a shade less ridiculous, but... *shrug*. That's not really faster than 68k software already can run on the best PowerMacs under Classic, so I'm curious how large the software base is that really *needs* the accelerator.