I suppose all they do is crack the existing ring gear with a chisel to get it off, heat the new gear, and slip it on.
Your missing tooth will most likely turn up.
Be a good option to replace the starter, and check why this has happened in the first place - I'd think the ring gear is reasonably hardy otherwise.
The original ring gear might be off alignment, starter overrun clutch might be seized, etc.

Yeah, it a PITA job, but get it sorted, then at least it won't be in the back of your mind when you start the car every time.