I understand why someone might not want to start there, but I think that could be a bit short-sighted. I think people are far too quick to dismiss code that's old. Yeah, it may have a lot of warts (for a lot of reasons, some more valid than others), but that doe not negate the value of understanding how something was implemented.
I'm also not suggesting this would be easy. There are some very complicated C codebases out there. I'm just saying that I disagree that there's no point in examining them.