In the meantime, use an issue tracker to keep a handle on all the stuff you actually need to do to bring this code up to speed. If a test needs creating, add an issue for it. I'd personally not keep this in a tracker that your manager uses to track metrics, but keep it separate and build it up.
If it's really a rewrite job, you'll end up with a lot of entries in there of varying size and complexity. You can start moving those issues across to whatever issue tracker your boss does keep an eye on as you get to them.
That way, you're not just walking in, announcing that the entire lot is garbage, and wasting time rewriting it all (which is what the business will think you're doing if the current implementation is at least vaguely functional), but you're gaining familiarity with the code and compiling a decent sized todo list that you can present at the appropriate time, as well as a list of fairly quick wins that you can get done when you have some downtime.
If it's really a rewrite job, you'll end up with a lot of entries in there of varying size and complexity. You can start moving those issues across to whatever issue tracker your boss does keep an eye on as you get to them.
That way, you're not just walking in, announcing that the entire lot is garbage, and wasting time rewriting it all (which is what the business will think you're doing if the current implementation is at least vaguely functional), but you're gaining familiarity with the code and compiling a decent sized todo list that you can present at the appropriate time, as well as a list of fairly quick wins that you can get done when you have some downtime.