The source for version 7 of J is available (circa 1993). I can't seem to find a link to it, but I have a local copy. My contact info is in my profile - I'll try to find the official mirror of it.

I was able to get j7 to build on OpenBSD (with a bit of tweaking), FWIW. K/Q seem to be much more my style, but I haven't had any success there. To be honest, I'm more curious about how they're implemented, though - I may do my own eventually.