More nostalgia!
I still have my ZX in a crate in the basement. Its monitor output is for an analogue TV and I have no idea how to make it work with a modern TV or monitor. I might do a bit of searching online…
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I was Team Acorn back in the 80s — two Electrons, then a BBC Master 128. Alas, that final set-up ended up going to the highlands of Scotland with a professor from one of the universities up there after I admitted defeat and joined the PC world. 
I’d love to have it back again, I have so many fond memories of games like Elite and Repton, and hacking them to run from disk instead of cassette.
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One possible starting point is the RGB to HDMI project. This is a hardware/software project involving a Raspberry Pi and some custom circuity for reading video output from old computers, generating a clean HDMI signal from them.
You can get pre-assembled boards from the Retro Hack Shack site (popular among retrocomputing enthusiasts). According to that page, the board, plus the analog video adapter will support the Spectrum +2A, +3 and 48K models.
These aren’t cheap, and the Raspberry Pi isn’t included, but it’s a Swiss Army knife of retrocomputing video, good for just about any old computer you might come across.
If you don’t want to tinker with your device but just want a single-purpose device to convert analog video (PAL or NTSC composite) to HDMI, you can get devices for that a well. One that popped up after a web search is: StarTech S-Video or Composite to HDMI converter.
More web searching was able to find devices with built-in tuners that (hopefully) should work with devices that generate RF video output, without needing to open the device to locate the pre-modulated video signal.
Good luck.
Thank you - that helped me understand the issues.
I found some detailed tips here:
However the main diagram showing the modified circuits is missing. I will try contacting the author.
Another option could be look for estate sales (not sure if that term is global, I mean when belongings of people who have died are sold off) or neighbors who are likely to have obsolete equipment hanging around (say, empty nesters or long-time retirees). It might be possible to buy an analog TV or CRT computer monitor for a very low price.
I bought a Timex/Sinclair TS-1000 with the money I earned doing farm work as a kid. While the built-in BASIC was great, I had the most fun writing Z80 assembly-language code, to get the best speed out it. Somewhere I still have reams of hand-written byte code I manually typed-in.
If trying to get it to display using a modern monitor is more trouble than you feel like going through, there are several good emulators out there. I’ll bet there’s even one that will emulate it entirely in a browser using Javascript by now.
I also bought one of those as a kid. It cost $100, which was pretty much all of my savings at the time.
I was not impressed. I had already done some BASIC programming on my school’s Apple II+, which was much much nicer. The Timex was very slow, unless you gave the command to turn of video, but then you couldn’t see anything. And the membrane keyboard jammed on the first day.
We returned it to the store. After some discussion with my parents, we went to Radio Shack and got a 16K TRS-80 Color Computer. It cost $300, but my parents kicked in the difference. That computer was so much better than the TS-1000. And I used it for many years, until we eventually got our Apple //c.