If you mean mint as in like new / concourse, then it's all got to come out and every part meticulously cleaned, painted, powdercoated, polished, replaced as necessary. While it's all out, the bay will need a fresh coat of paint and the subframe stripped and powdercoated.
I'm building up for a full on restoration of my engine bay next year. With the time I reckon I'll need or be able to spend on it, I can't see my car needing a tax disc at all next year.