If the original developer is still around, have some informal chat here and there on why the worst part are the way they are.
Frame it correctly: "I don't understand why these classes share this data or code, do you remember why?" is miles better than "Explain me this mess please it's making me crazy"
If the original dev is still around focus on the task that come from business so he's free to focus on fixing the structure, he is, after all, the local expert on it.
In general a rewrite is not something you'd do without having a clear understanding of the requirements. What you can do instead if you don't have the dev support is to fix thing as you go, removing cross dependencies and encapsulating behaviors. You don't have to fix code only to hide the ugliness behind a clear structure and as small as possible interfaces, order will eventually emerge.
Frame it correctly: "I don't understand why these classes share this data or code, do you remember why?" is miles better than "Explain me this mess please it's making me crazy"
If the original dev is still around focus on the task that come from business so he's free to focus on fixing the structure, he is, after all, the local expert on it.
In general a rewrite is not something you'd do without having a clear understanding of the requirements. What you can do instead if you don't have the dev support is to fix thing as you go, removing cross dependencies and encapsulating behaviors. You don't have to fix code only to hide the ugliness behind a clear structure and as small as possible interfaces, order will eventually emerge.