If possible, you could try to get the requirements and design to those and write something new.
If that’s not possible, I would start flow-charting. I always do it by hand in whatever tool you have handy. Some languages have tools that can do this for you (Go and Java and I think PHP to name a few) if it’s that big of a mess. From there, find your inputs and outputs and go back to the “rewrite it” part.






My company GitHub has copilot do code reviews. That is the extent of my AI use. I’ve had to correct copypasta from AI agents that coworkers used that was just wrong.