I'd say the most practical route, if you're happy with the sort of clarity you're going to get from composite, would be to look for a composite 9" monochrome monitor (like a surveillance unit) to steal either the analog board or the entire guts from. (If said monitor has a tube with a short enough neck and is otherwise dimensionally compatible I'd say "all the guts" to avoid trying to figure out how to change the yokes, etc.) Picture quality will of course leave much to be desired. The other option (that involves staying CRT) is to swap the guts with a 9" Monochrome VGA monitor. A Raspberry Pi doesn't do VGA natively and I *seriously* doubt one of those HDMI->VGA dongles is going to know how to handle a mono monitor correctly so that's not really very optimal either, although you might be able to work around it in software. What are you intending to use the system for? If it's just for an MP3 player or something composite should be fine, but it'll suck if you want it to do computer things.

Hacking the stock monitor to handle either NTSC or VGA frequencies and make it do grayscale is a pretty serious effort. Here is one of the very few projects I've seen that use the original guts, and it depends on custom hardware hacking to both the analog board and an ancient VGA card with a digital feature connector.