Napoleon's only been gone for about two hundred years, whereas Common Law has some real classics. For example
https://en.wikipedia.org/wiki/Statute_of_Merton was a set of laws promulgated in 1235, some of which remained in force (at least nominally) until the 1980s. I don't know much about Canon Law, but that surely goes back even further.
All that to say, you can just do your best to understand the law in the the context in which it was written, and replace the text every now and again.