Two months later I realized I was painfully and slowly following in my predecessor's footsteps.
Absolutely true: there's often some pearl of hidden wisdom that justifies a software being perceived as bad. If it is you who wrote the software in the first place, you might be able to fix it the right way, or at least to leave tracks (comments?) for the ones who come after you. If you're not the one who wrote it, and there's no track, you are pretty much doomed to realize the hard way.
On the lucky case, the ugly part is covering some issue which is no longer a problem nowadays. I'm now thinking about, autoconf, that makes sure you've got uint32_t defined: what libc doesn't come with uint32_t defined nowadays? Yet, replacing autoconf means you've got to ensure portability yourself: good luck with that! Or get rid of portability entirely: thank you very much for restricting those who have a different system than yours.