Replacing the fork and going threadless is doable, but a few things to be aware of. Choices of headset and stem will be limited vs. current 1-1/8 standard. You will probably want to trim the steerer tube of the replacement fork. It is tricky to make a square cut unless you have good tools. Last thing is you could be sinking a bunch of money into a lower end Peugot. Me, I would be shopping for another bike.