So instead of preserving what's left file the stub perfectly flat and square, without touching the flat spot that keys the spark advance, find the center and carefully punch it. You can check whether you're centered by turning/cranking the engine over and watching the punch mark. If it's centered it won't move, if it isn't you'll be seeing a circle. (A light punch mark can be adjusted if it's off center by punching right next to it and letting the punch forge material into the first hole) Use a small drill to start with and have a helper, with a good eye, tell you whether you're going straight in on the plane that you can't see. It'll help to remove the front wheel and fender, or go all out and remove the forks ( Now there's another can of worms ). Slow and steady does it because a broken drill bit is far harder to remove than simply drilling a straight hole. Use cutting oil or at least some sort of lubrication to keep the drill from biting and breaking off, ditto for when you tap the hole. Work your way up to the right size, tap it and either install a stud or just use a bolt. Pat yourself on the back for doing a nice job. Pictures for the rest of us would be great too.