How old is your Peugeot? Until the early 80s Peugeots used metric diameter tubing and metric (aka "French") thread. As others have noted, the fork is probably metric thread, so unless the headset is shot or you have a burning desire to sink a lot of money into a new fork/headset/stem/bars (yes, the stem diameter and bar clamp diameter are also metric) you may want to hang onto those pieces at least. The BB shell is also likely metric, but you should be able to find a square taper axle of the proper length to suit a fixed gear conversion using your existing cups.

By the early 80s most French manufacturers switched to English threading, so you may luck out depending on the age of your frame, but the Nervex lugs argue against this possibility.

Keep in mind that high-end French components (Simplex LJ derailleurs, Ideale sddles, Maillard Professional hubs, etc.) are quite desirable for some collectors, particularly the well-heeled Japanese ones -- so don't toss them.

The PX-10 frame is regarded as one of the best riding production frames made, so IMHO at least it is worth hanging onto, even as a conversion. And please don't hack off the braze-ons and/or derailleur hanger or repaint it if the finish is at all reasonable. It will ruin the value of the frame should you ever decide to sell it. The metric nature of the beast means it will mainly appeal to a fairly small market already, and changing it from its original configuration/finish will likely lose even that market.