I have decided to put trekking bars on my Peugeot UO-18. More hand positions, upright posture, etc. I know I will have to replace the stem but I don't know what diameter I need. Can anybody help? I don't have any calipers or convenient way to obtain any to measure the diameter of the head tube. Thanks in advance for all the help.

Most trekking bars required a stem with a 25.4mm clamp - standard for MTB's. If you are putting into a 1" threaded headset, I would recommend using a Kalloy adjustable stem. It lets you raise/lower the stem, but also has some fore-aft adjustment. Good for touring/trekking so you can adjust it to an upright position for some days, and a lower position for days when you've got headwinds (and they will come).

Thank you, HartleyMartin. that pretty much answered my question and I found what I need on Ebay.