The general standard is 25.4 or 1 inch so you should be able to thread a dropbar through it; just make sure the bar you buy is 25.4 as some also come in 26.0 and many of the newer bars are 31.8 (and stay away from Cinelli handlebars). Obviously you can get better prices used..

Edit: also make sure the handlebars you pick are barend shifter compatible; a new one will say so, otherwise you'll have to check that the end section of the bar has a wider ID than the rest. Also if you want that classic road bike look with the stem parallel to the ground you will need a new stem.