Do I need to replace the bolts or use new hardware when installing this?

The gasket itself does not include fasteners. Reusing factory header bolts is standard practice — just ensure they're clean and torqued to spec per your service manual.

Should I use sealant or adhesive on the gasket surfaces?

Pre-formed gaskets work best dry; applying sealant can cause the gasket to shift during installation. Clean both mating surfaces with a wire brush and install the gasket directly.

What happens if I reuse an old gasket instead of replacing it?

Old gaskets compress and lose their seal integrity. A fresh gasket is cheap insurance against exhaust leaks that develop over time and rob you of performance and fuel economy.