Is the Amiga chipset so complex that it can't be replicated directly rather than via an FPGA? I have limited knowledge of hardware engineering - but I imagine with modern manufacturing methods it would easily be possible to build the AGA chipset into a single chip. However, this would likely require extensive knowledge of transistors and to have it manufactured it'd probably need to be ordered in bulk to be cost-effective. I imagine this is why FPGAs are used rather than actually doing it like this.
The 68k CPUs however should not be difficult to get at all - cannibalizing old 68k macs would be quite cost-effective as there are probably millions in old US educational warehouses - I was able at one point to get over 500 computers used in a school system in Petersburg, Virginia for the cost of hauling it away - sold it to a computer recycler for scrap metal and made a decent killing on it. Lots of 68k and PowerPC macs I had in that stock.