Will this gasket work with an aftermarket downpipe?

Only if your aftermarket downpipe uses the same two-hole bolt pattern as Honda OEM. Measure your bolt spacing before ordering — aftermarket units vary by brand.

Do I need any additional hardware to install this?

You'll need the bolts that came with your downpipe or header. This gasket is the sealing component only — fasteners are separate.

Can I reuse an old gasket if I'm just replacing the bolts?

No. Once a gasket is compressed and heated, it loses its seal integrity. Replace it every time you separate the header and downpipe connection to avoid exhaust leaks.