Re: 98% ZX Spectrum look a like
Most ZX Spectrum ULA chips can be tapped to produce a composite video signal.
It takes a bit of soldering, but it can be done, and there are FGPA versions of the ULA now if you really want to custom-build it.
Even the original RAM chips have probably died by now and are easier to just replace with one large chip than the multiple Upper/Lower RAM chips of the original.
Basically a Z80 of any fashion, maybe one or two custom chips / RAM chips, and you could easily do such. The expansion connector was literally just the Z80 I/O pins for certain ranges brought out to the connector, IIRC (even the Interface 2's ROM cartridges were nothing more than a direct memory chip sitting on the right exposed address lines).
The problem is that almost all the components fail over time - the ULA, the RAM, the keyboard membrane, etc. and they are what made the Spectrum unique. The bits that don't fail (the Z80 itself, that's about it) are off-the-shelf and bog-standard.
It would be easier to just ditch the entire interior and stick in a Pi Nano or Arduino. With the latter, you could easily interface with a real Z80 (timing would be a pain in the arse but manageable), and expose the I/O lines directly for peripherals.