I was thinking about this, and, in this thinking, I came to conclude the lack of documentation is a major roadblock in mantaining obsolete systems. Sure, one can poke and probe and maybe eventually trace a fault. But sometimes, without docs, you're lost and hopeless. Shamefully, SiliconGraphics systems are scarcely documented.
Simple parts and boards can be fabbed, and new prototyping technologies are taking that further, however, we have not a chance of replicating the functionality of custom ASICs that weren't documented, even less so if the subsystem in which they're integrated is also a black box. Big multilayer PCBs are also still beyond reach of the poor hobbyist, so too bad for us.
Indy R5000, 256MB, XL-24, IndyCam
Impact10K 195MHz in teal case with purple bottom, missing several bits, awaiting completion
misc: Some Sun stuff, lots of x86 crap, one MSX and a MSX2+, a ZX Spectrum clone, slotloader 400MHz iMac G3