I'd try and fix that one first. You may need to take the cone pulley off the spindle first, but you may be able to do it on the spindle if you can find out how thick it is AND you are very careful. All you need to do is drill and tap where the tooth should be for maybe 2-3 8-32 screws, then carefully grind them into the same profile as the other teeth. If you want to get fancy you can build up around the screws with JBweld first. It'll look a little prettier but won't be functionally that different. If you're worried about that tooth affecting your back gear, use brass or alu (if you can find them) screws.
Perfectly valid repair and no real harm in trying as long as you don't drill through into your spindle (hence, be very careful). It's not like you can screw that missing tooth up any more than it is already is it?