i absolutely love french bicycles. you should too.

bottom bracket -- if there is a functional bottom bracket on the bike currently, keep it. you should be able to adjust chainline either via spacers on the bracket itself or with spacers/putting your chainring on the inside. also, depending on the cups that are on the bottom bracket currently, you may be able to buy another, shorter, spindle to accomodate whichever cranks you want... example... i have sugino french threaded cups and have a sugino 75 njs spindle that i use with it... for my sugino 75 cranks.

headset and stem -- if the headset included is in good shape, keep it. my gitane has a stronglight competition headset that works like a dream after some new bearings and a lot of phil wood grease. of course, with an original headset and fork, you are possibly stuck with the 22.0mm instead of the standard 22.2mm steering column.... as sheldon brown has pointed out, sometimes the fork itself is 22.2 and you sand down the headset rings.. if that doesn't work, it isn't that much work to sand down a stem to fit. you will likely not suffer much aesthetically if you finish with a fine-grit sandpaper and only sand up to your insertion point. it took me 30 minutes while watching an episode of the O.C. that bike you see in my avatar is french -- that cinelli stem is sanded way down.

now, if the headset is a total piece of crap -- or if you simply want to replace it, get the stronglight A9 M25x100 (french threaded) headset. they still make it -- it uses nice needle bearings and is the smoothest french threaded headset that you can find. www.xxcycle.com has it for $24.79 + $10 shipping to the united states ... this is WELL under what harriscyclery charges here in the states. with the A9, assuming that your fork steerer is indeed 22.2, you will have no problems with stems. of course, it might be 22.0 and you'll still need to sand.

you should do the conversion.. it's fun.