My last one blew up a few years ago when I tried to see if it would still work (capacitors in the supply had gone, took the board with it). So this project is really tempting for me, I'll probably see if I can get this to work.
FPGA's are interesting, a kind of half-way point between software and hardware.
Rather neatly, you could chuck a second CPU on the end of the bus and just use the host machine for IO. That made the machines EXTREMELY fast for the time. Many second CPUs were developed with different architectures from 6502, Z80, 32016 and the original ARM CPU (which was developed as a slave of a BBC micro!). I myself have owned a 6502 unit.
You can still get NEW tube second processors including a 64MHz ARM7 with 64MB RAM ( http://www.sprow.co.uk/bbc/armcopro.htm )
Pretty awesome for a 1980s 8-bit computer eh?
More info: http://en.wikipedia.org/wiki/Tube_%28BBC_Micro%29
There were several options for that, see http://en.wikipedia.org/wiki/Tube_%28BBC_Micro%29