Hi Philip,
Not sure how much of a copy of the South Bend lathe your Sabel is. I can tell you that one broken tooth in the back gears will do no harm at all on the South Bends. I have a much battered 1949 SB 9" lathe with flat belt drive and before I owned it several teeth were broken off the back gear with no real harm done (as long as they are not adjacent teeth). The only sign I see of the broken teeth is that back gear is not quiet. I regularly tap up to 1/2"-13, and drill large holes up to 3/4" in steel in back gear with no trouble. I wouldn't fret about the teeth unless the gearing is actually causing a problem for you.
You might check the South Bend forum on www. practicalmachinist dot com for info on spare parts. If the S&B machine was a true copy of the SB, an SB backgear might fit, and there are guys on the SB forum in USA who can supply them.
JD