Well I gather the XScale is a descendant of StrongARM technology, so it's pretty efficient. And those ones are available at up to 600MHz, run ucLinux and have up to 204 pins of I/O (not sure if any of those are doubled up). So it should do the trick nicely 
Running an assembly language emulator directly on the CPU would of course be the fastest, but the hardest to write. Perhaps easier would be running a stripped down ucLinux and an open-source emulator, with fresh code to do the pin I/O trickery.
In the Lost Pages of the MLA, the Gumstix came up in a thread about adding wireless to a Duo. (See their accessories page), but it never occurred to me to use them as a CPU upgrade as well.
Ideally you'd make the emu as modular and portable as possible, so people could emulate whatever old CPU they want
Running an assembly language emulator directly on the CPU would of course be the fastest, but the hardest to write. Perhaps easier would be running a stripped down ucLinux and an open-source emulator, with fresh code to do the pin I/O trickery.
In the Lost Pages of the MLA, the Gumstix came up in a thread about adding wireless to a Duo. (See their accessories page), but it never occurred to me to use them as a CPU upgrade as well.
Ideally you'd make the emu as modular and portable as possible, so people could emulate whatever old CPU they want