https://groups.google.com/g/neonixie-l/c/5eZbhpJCeTM

Is there, yes
Is it reasonable? Not really, not that that has stopped people.

Historically a so called video transcoder would be used that would first convert the signalling type then refresh rate.
There are modern devices that convert between many video standards but ntsc to 18khz TTL will be a cludge as neither are really a typical gaming standard.

In a case like this it’s worth noting that amber composite screens existed that came in either or both signal standards. (NTSC & TTL) right from the factory with no mod.

First what model/brand/type of screen is this? (Your screen may have unpopulated circuitry for composite)

https://m.youtube.com/watch?v=yItzHekCygo

What system will you actually use it with?
(Many 8 bitters like the c128 and others have existing solutions for some type of TTL adapter)

Depending on the actual screen brand model you will have different options.

What is the real goal here, you wanna see output from those machines on the cheap and you already have an amber monitor, or you think they'd look really cool if you got one?

Because cheap way to get composite CRT if the console crowd are buying up all the color CRT with composite input locally for $$$ these days is to look out for an old "security monitor" also some of the late mono and mini 5" TVs that were a fad due to being cheap and last blast of CRT companies around millenium have a composite input. Though they have been faddish too recently with ppl building PIs and bitbanged arduino composite projects into them. So prices on those might not be too good either.

Now the way to get a cheap but good RF convertor if you didn't mind the footprint was to pick up a $5 VCR with composite inputs, which would then through some setting put it through RF on whatever analog tuner TV you could find, but last couple of years VCRs going rare plus $$illy. But there are also still RadioShack modulators turning up in thrifts once in a while and the sega genesis ones are hackable too. Don't be fooled by the promising nintendo grey box, it's just an RF switch.

It's possible to "make" an amber or green monitor by mounting a coloured sheet of plastic in front of a b/w CRT.
This worked especially well with monitors which had an anti-glare or anti-radiation shield in front of the monitor.
You know, those boxy monitors where the CRT was not sticking out of the frame of the monitor chassis.

What kind of 'old Amber Hercules monitor' exactly?

Does it use TTL (DE9 connector) or composite (RCA connector)?

If composite and it accepts the sync output by th 8-bot consoles you just need a suitable passive cable.

Both C64 and Atari 800 have (slightly different) DIN connectors with composite signal onboard.

If it's composite but needs different sync, you are in for a bit more fun.

If it's a TTL monitor, you'll need some interesting circuitry to take composite or S-Video from the 8-bit machines and converting that to something the monitor understands, and even if you can find a schematic, image quality may suffer. Here's a USEnet thread of someone sort of getting composite output (from a Raspberry Pi, although that shouldn't matter here) to work on a TTL mono monitor.