Daytime you should have no issues. There isn't a simple answer regarding nighttime temps, but if there isn't a lot of airflow, it could get pretty hot in there. Think of a car with only partially open windows on a warm day. Temps could rise considerably and be held in at night when the coop is...
The other thing I can think of quickly (and depending on how in depth you want to get with re-doing the ventilation), is to add dormers with operable windows. This would require additional framing, cutting a hole in the roof and flashing/roofing again on that side, and added cost in windows of...
Once you get the roof ventilation squared away, you might also consider an inline fan if you have electricity nearby. AC Infinity makes a nice one. You can even attach a charcoal filter to it. You would need to buy a grate cover for it, so that no chickys or critters touch the fan blades of...