I decided I wanted to have a go at interfacing some of my retro kit to a Teletext (Ceefax) adapter and found "The Teletext Experimenter’s Board" by Oddbloke Geek. This board uses the SAA5246AP Teletext processing chip, so I bought a couple of those and am planning to build my own circuit. One addition I want to make is a composite output; the 5246 only outputs RGBS.
There's a number of classic converter chips from the 80s and 90s; parts such as the AD723, AD724, CXA2075M and MB3516A. All these chips are still available, although the AD72x ones are getting relatively expensive.
While searching for options, I came across this reversing camera CAN bus module on AliExpress - it's an RGB to composite converter, and works out at £11 shipped, so I bought one.
Inside is a neat RGBS-to-Composite (NTSC) converter board, which I will remove and repurpose, plus there's the bonus of a project box + connectors for something later! This board uses yet another different chip - the KA2198BD. This chip and the CXA2075M and MB3516A are broadly pin compatible and interchangeable.
I've yet to hook up the board, but it looks like it will do the trick, and will save some time sourcing individual components and building a circuit.
Now I just need to find the time to work on the teletext portion, and then see how to hook it up to something such as a Raspberry Pi or retro computer.