You'll need a new grommet. Grommets come in all sizes so if the hole is too big for the size that was there, you can go up one size. There are different grades and thicknesses of metal as well, so look over whichever ones you're thinking of getting. Marine stores are a common source, as is the net. Nickel-plated brass should resist tarnishing a little better than plain brass. I think I've even seen stainless steel grommets advertised, but have no experience with them. For a stronger hold in thicker material, there are spur grommets where the two halves mesh a little more securely. Each size and type (plain vs spur) requires it's own matched tool. There are pliers-like tools (and fittings for standard pliers) for lighter duty, and an anvil style where you hit one piece against a heavier anvil, using a hammer.
I've replaced and installed new grommets in wings and other things where needed. It's not difficult to do a passable job. Depending where the grommet is, it may not really be needed - if it's a mounting hole that's sandwiched between two flat surfaces such as a plate and washer, it's kinda redundant to also have a grommet. You might even get a stronger joint w/o the grommet, since the force will be spread over a large diameter than just the grommet circumference (assuming your washer is big enough).
