My Old Town doesn't have a keel, so I'm working off old memories here, but aren't the keel screws just put in using standard brass finishing washers? If so, you can get them at most hardware stores to fit various sizes of flat or oval-headed wood screws. It's not a bad idea when you go shopping at hardware stores to stick a refrigerator magnet in your pocket before you leave the house. A lot of the common "brass" bits and pieces that they sell are actually coated steel and the coating won't last long in a marine environment. Check each piece with the magnet before you buy it because it's not unusual to find a mix of real brass pieces and coated steel pieces have been stick in the same bin. If it sticks to the magnet, you don't want it on your boat.
Copper roves tend to have a fairly small hole in their centers and unless you were to buy very big ones they probably won't take a screw bigger than maybe #6 or so. When used for real riveting you want to be able to force the rove down over the rivet's shaft, stretching the hole and causing it to stick in place long enough to get the rivet trimmed down to length and properly peened. To use them as washers for screws you would likely wind up holding them with a pair of pliers and trying to drill out the holes to a larger diameter first, which wouldn't be much fun. I doubt they would be a great choice for this job.
Rivets and roves are certainly worth knowing about though. I'm surprised that they have never really filtered down into the canoe industry. They're stronger than screws and lock the pieces together, they require a smaller hole in the wood than a screw, leaving more wood for strength and they're fairly easy to remove, replace or tighten when needed. One of these years I'm going to use them for installing gunwales on a canoe. They worked pretty well holding boats together for the Vikings!
