Do I need new hardware to bolt the header back down?

The gasket itself comes alone — you'll reuse your factory header bolts. Inspect them for damage or thread wear before reinstalling. If any are corroded or stripped, grab new hardware while you have the header off.

Can I reuse the old gasket if it looks okay?

No. Header gaskets compress and degrade every time they're heated and cooled. Once disturbed, they won't seal the same way. Fresh gasket every time you pull a header is the safe call.

What prep work is needed before installation?

Clean both the header flange and cylinder head mating surfaces with a wire brush or gasket scraper to remove carbon and old gasket material. Dirt trapped under a new gasket will cause leaks. Let surfaces dry completely before seating the new gasket.