Save that stem! You'll maybe be tempted to get something new, but if in fact it is French threaded the stem will have an insert diameter of 22.0 instead of the standard 22.2. That's really the only tough to find (and not really, if you are patient) French part these days. There's a possibility you will need a 25.0 clamp handlebars (as opposed to the more standard 25.4 or 26.0mmif it is indeed a French stem...so keep that in mind too. Like mentioned, the threading issue is nowadays a non-issue thanks to Velo Orange....and everything else related to converting to fixed gear is the same as any other vintage frame.