First, I forget, the insides of your walls are insulated, right, not bare stone or cinderblock or concrete? If bare stone or cinderblock or etc, then your best bang-for-your-buck will be to cover it over with plywood, with or without additional insulation behind it. To prevent condensation. Stone/block/etc walls are
notorious condensation factories during warm snaps. If there is no exposed stone/block/etc in the inside, then ignore me here
Do the gable ends of the building have any vent openings built into them, even if they are into the attic rather than into the coop space. If so, use those openings -- connect with coop ceilings using ductwork, which ought to be insulated (condensation, again).
Otherwise, if the walls are wood, you just have to cut vents in them. I would NOT advocate pvc pipe or anything like that, b/c you just don't get very much airflow. It takes, uh, I can't find my calculator but something on the order of 10-12 4" circles to give you just ONE square foot of ventilation... and you will prolly need more than that
Nicely made vents will not spoil the building's look, and besides, if animals are being kept there, function is more important than pretty *anyhow*.
If the walls are all stone or cinderblock or whatnot, and you can't or don't wanna go putting holes in them, you might have to use the windows. Which is not ideal at all. The best compromise would be to remove the windows and replace with shorter ones with a transom-style vent above them in the original window opening. If you don't want to do that you could construct a plexiglass-and-wood-frame baffle that fits on the inside of the windows, like a6" deep box that is open only at the top, so that when the window is cracked open the air actually enters the coop near the ceiling, you know? This would be a pain, but you NEED ventilation.
I forget whether you have droppings boards cleaned every morning -- if not, doing that will considerably reduce the moisture load while you work on ventilation.
(e.t.a. - with respect to fans, they make agricultural fans designed for heavy-dust environments. However they are expensive and, more to the point, dunno whether you could actually obtain one that would adjust small enough for such a small building. But if you could, I suppose it is a theoretical option)
Good luck,
Pat