The headset is most likely just fine, and prolly just needs disassembly, a good cleaning and greasing, and reassembly with proper adjustment.

What to replace? Mafac Racers are pretty good brakes, I would leave those but add salmon coolstops for a little more grab.

If the Simplex Prestige derailleurs are working and not worn out, I would keep them for now, and start searching for upgrades.

Replacing the original cottered cranks and steel chainrings is a good upgrade. You can keep the stock cups and just replace the spindle and the cranks/chainrings. According to Sheldon, all 3 series bb spindles have the same bearing spacing. Most have the same left end length, and it's the drive end length that varies. My bike has a 3T spindle, which has a typical 32mm left end length, and a drive side length of 39mm. With this spindle there is 10mm clearance between the chainwheel notch on the right chainstay and the chain on the small chainring. This is a bit much, and requires the FD to be adjusted out quite a bit, but works and shifts fine. drive end lengths of various 3 series spindles:

3T = 39mm
3S = 37.5mm
3N = 36mm
3P = 35mm
3K = 33.5mm
3H = 30.5mm

These all have the 32mm left end length and 52mm middle width.

So with a 3P spindle I should only have 6mm clearance btwn chainstay notch and chainring.

What is the stock distance between the chainstay notch and inner chainring?


Also, does the bike have the original steel rims? Alloy rims would also be a nice upgrade without too much $$, plus it gives you the opportunity/excuse to re-lace, new spokes, nipples, new rim liner, new tubes and tires.


And of course, all the greased bearings - bb, hubs, head, should be well cleaned and regreased and properly adjusted.

And all the cables replaced & adjusted & oiled of course.