First, your Virago doesn't have a magneto. It has an alternator, and yet there is supposed to be oil in there with it. If you replaced the Black Box and your battery is perfect, and you are sure the wiring isn't fried or a fuse cooked, I'd say your coil is bad. It's just a guess, but that is really the last link left in getting a spark on the plugs. I'd look at the coil if it was mine.