Do you have time/energy to do both Zig and C, side-by-side? Could be some useful results from that, both for you and others, and if you start seeing problems on the Zig side, you've got a fallback.

Is it really just the code, or a whole system? If a whole system, check and/or create a high-level overview, even if it "should be obvious" and make sure you have the documentation necessary to make a change, rebuild, test, distribute and install. (I.e. you have to go through that process to be sure it works). As for code, depends partly on how much expertise in the coding language(s) you think will be available if a change is necessary, particularly if people have used unusual language features or coded in an unusual way compared to "normal industry practice" - perhaps make a few general remarks noting such stuff.