I have found that implementing a regression test suite for such systems is a really good way of holding a mirror up to the mud ball and showing it what it could

I have found that implementing a regression test suite for such systems is a really good way of holding a mirror up to the mud ball and showing it what it could look like if it smartened itself up a bit. If it’s too much to apply some discernment and modularity to the underlying codebase then you can instead do this with the test suite and reflect those discoveries back into the codebase with the benefit that you can now prove your changes don’t break the documented, expected behaviour. It’s like rewriting the code but by passing the ball from production code to test code and back to figure out who the individual players are.