1. A plug repair can be made to get you home or to a shop, but under no circumstances should you consider that to be a lasting repair. Even in a four wheeled vehicle, plugs should be considered temporary emergency repairs only.
2. A proper repair involves removing the tire and patching it from the inside. This process is pretty reliable if done properly with fresh patches and adhesives, and I wouldn't have any qualms about doing this on a car or truck. On a two wheeled vehicle, however, I side with those who say no patches.
3. In theory, most Harley bias ply tires will accept tubes and you could repair the leak by installing a tube. However, this will add weight and will cause the tire to run hotter as well. Since you have to break the tire and wheel down to install a tube, it makes more sense to just replace the tire and keep all the benefits of a tubeless tire.
I know it really irks a cheap guy like myself to throw away an almost new tire, but you have to ask yourself if your life really isn't worth a couple hundred bucks. My answer to your question: Replace the tire!
Jerry