Easily done. Just put a dual-filament style socket & amber bulb in them and wire as a running light/turn signal. Would look better than a modern LED IMO...
The problem with LED bulbs is they are very bright when looking at them straight-on, but don't put out much light at all from the side like a traditional filament bulb. If you do want an LED to plug into a regular 1157 socket, Speedway has a 1" diameter one and sells them here, but like Earthman said they would point up, giving you the side-lighting LED issue - http://www.speedwaymotors.com/1-Inch-LED-1157-Light-Bulbs,7767.html