AFAIK the framework needed for a modern x86 CPU is a major pain in the *ss. You'll need lots of stuff like a north bridge (RAM controller, interfaces) and high performance power regulation. To make the north bridge work, you'll have to license a complete PC BIOS, which in turn requires more peripherals, so you end up with 3/4 of a PC mainboard.
The easiest way would be to take a small (µATX, mini ITX) mainboard and build a PCI <-> Zorro III or CPU-fast-slot bridge and just connect the mobo to the Amiga, just like in Ratte's link - would even give you an upgrade path, so you might consider using PCIe instead in the first place.
Again, this is pretty much an Amiga on a PCI card. If you also use the x86 side's graphics hardware through RTG and the sound through AHI, there's actually not much left of the original hardware, so you could also use UAE with Keyrah and Catweazle.
