There's no mention of speed grade, so presumably the 50ns variant is plenty fine. The character video memory scan clock isn't going to be all that high. Maybe ~2MHz, so even anything less than 200ns or so should be killing it? Just a wild guess based on an NTSC/PAL clock divided by 5-6 for a character clock. (But I could be wrong, it's been ages.)
I'd take that schematic and reenter it in KiCAD. Then modernize the memory, no need to have a pile of 2708's and whatever they used for RAM, just drop in bigger memory parts and rip out the various extraneous support logic, like the superfluous address decoding. (Like maybe a 27C256, 32k*8, as EEPROM. They can still be found.)
I didn't look closer, but what kind of keyboard do they expect? Maybe rework it so it has a PS/2 port (which is really little more than a shift register).