You may create a new system from scratch and write the new features there, while temporarily leaving the mess where it is. The team will be able to try out how

You may create a new system from scratch and write the new features there, while temporarily leaving the mess where it is. The team will be able to try out how good programming can be and perhaps support you more. From there you should gradually move the old features in the new system. Even if you were to never fully complete the refactoring the situation would be much better.