Nick, based on that measurement between teeth, you have a 139 tooth, 6 V flywheel. That was my picture in Bill's tech tip.
The wear on the teeth isn't that bad, IMO. Unless the starter is not engaging, I wouldn't worry about that.
Guys here have said that you can run a 6V starter on 12V without problems (if you do go with 12V). I haven't tried that myself, but don't doubt that it's true.
