Both government regulators and operations personnel have made careers out of trusting black boxes that no one fully understands anymore. Regulatory specifications are ambiguous and disorganized. Imagine working with someone who knows that you are trying to shed light on the technology that they were responsible for, yet do not fully understand. How cooperative do you think these people will be with your efforts? How do you think the managers responsible for this work survive the politics of the project's inevitable failure? Blame rolls downhill. Modernization projects require strong leadership and collaboration across many teams. That's a huge problem because such an environment very likely does not exist in the world today. No one understands how the algorithms work or even how they should work! Forget how challenging it is to work with COBOL, which someone could eventually figure out. Whatever you've written has to be tested, and you don't have a trusted source of business logic from which to verify how it ought to work. You only have the black box.
If you've been given this task, start interviewing at other companies. You were given a suicide mission by people who are well aware of that.
My experience with this was with regulatory margin trading systems supporting a multi-billion dollar market.