If I understand correctly, you have a French B&W TV from the early 80s and want to use it as a display.

Your initial idea was to use the modulator inside a French Atari VCS. Since colour doesn't matter, it has chances to work, as what's matter is the norm used by the modulator (see my edited post below). Just feed some random 50Hz composite signal into the composite input of the modulator and you'll most likely get a picture on your TV. The ideal solution would be a perfect 75ohm monochrome signal (can be got out of an Extron RGB box by only using the green pin with SOG enabled).

Otherwise, get an old (prior to 1994) French VCR and use it as an L modulator itself.