I expect the stem on the Citizen will have snapped but if you're luck they'll be enough protruding from the crown to unscrew it. Do you know what movement's in the Citizen and what all the movements look like for general condition.

There are specific dies that will mostly fit this crystal shape, but I just find it easier doing them by hand. Occasionally the odd one can be difficult, there is a workaround, and it isn't by force .

How does the size of the new compare to the old.

As I wait for my first set of watchmaker screwdrivers to arrive, which is taking longer than I had hoped, I'll post a photo of the small stockpile I made to work on. I'll be starting with the Citizen: the missing crown is my doing. I unintentionally applied pressure from the side and off it came. I've put it aside, and I'm hoping it's detached rather than snapped, and without damage to other parts, but until I have the screwdrivers I can't remove anything to examine the stem section. Two of the Seikos are are working, quite well actually, although for the price I paid, none of them should really be in "serviced" condition, so after letting them run on one wind, I've left them unwound.  The Citizen was the cheapest, and so the one I could most bear to go wrong with, although all of them are rather nice watches. I think they're all from the late 50s to very early 60s, but I haven't managed to date them more closely than that. The Citizen and the Seiko Crown each have a small crack in the crystal, so I'll be hoping to replace those.        

Search for "Miniature 3 jaw chuck" on ali.