Its likely that every component of the engine has now been swapped out and almost no Q3 derived code exists.

Yeah I know, I work for Sledgehammer Games on cod. It's not that it's been swapped out, it's been rewritten and rewritten many times over the years. What I meant to imply (but didn't succeed in saying) was that for systems that were in q3a, if you knew the q3a code, then you would feel a sense of familiarity with those parts of the cod code.

I occasionally do some random spelunking for fun and while there's practically no code left, there are still a few small remnants like the odd comment or function signature (or partial signature) that are still there, and some that even date back to Quake 1!