That structure is a great find.
Here are some shed conversion coops to help inspire you:
https://www.backyardchickens.com/articles/the-mulligan.74743/
https://www.backyardchickens.com/articles/aarts-coop-page.65912/
https://www.backyardchickens.com/articles/the-scoop-on-a-rubbermaid-big-max-coop.76444/
https://www.backyardchickens.com/articles/toy-shed-conversion.64879/
I'm sorry to say it, but those little vents are nearly useless.
They give you square inches of ventilation when you need square feet.
https://www.backyardchickens.com/articles/repecka-illustrates-coop-ventilation.77659/
Soffit and gable venting is usually one of the best options for a severe winter area, but that structure may not be built in a way to permit the usual way to create that since there is no meaningful roof overhang.
You could put top-hinged vents right under the roofline on the front and back and as high into the roof peak as you can manage on the gable ends. I'm not at home on my desktop so I can't sketch it for you right now.
A couple useful articles:
https://www.backyardchickens.com/articles/aarts-extreme-weather-spiel.75893/
https://www.backyardchickens.com/articles/cold-weather-poultry-housing-and-care.72010/
Chickens wear built-in down parkas so they're highly tolerant of cold as long as they're kept dry and out of the wind.
Here are some shed conversion coops to help inspire you:
https://www.backyardchickens.com/articles/the-mulligan.74743/
https://www.backyardchickens.com/articles/aarts-coop-page.65912/
https://www.backyardchickens.com/articles/the-scoop-on-a-rubbermaid-big-max-coop.76444/
https://www.backyardchickens.com/articles/toy-shed-conversion.64879/
I also purchased some vents, I have two vents that are 10" x 10" and three that are 6" x 14" that I plan to put up high above the roosting bar. These vents are adjustable, fingers crossed that the ice and snow don't freeze them so I can't adjust.
I'm sorry to say it, but those little vents are nearly useless.

https://www.backyardchickens.com/articles/repecka-illustrates-coop-ventilation.77659/
Soffit and gable venting is usually one of the best options for a severe winter area, but that structure may not be built in a way to permit the usual way to create that since there is no meaningful roof overhang.
You could put top-hinged vents right under the roofline on the front and back and as high into the roof peak as you can manage on the gable ends. I'm not at home on my desktop so I can't sketch it for you right now.
Yes, I have read that, but I do worry about the extreme cold that we have and wonder how others handle this.
A couple useful articles:
https://www.backyardchickens.com/articles/aarts-extreme-weather-spiel.75893/
https://www.backyardchickens.com/articles/cold-weather-poultry-housing-and-care.72010/
Chickens wear built-in down parkas so they're highly tolerant of cold as long as they're kept dry and out of the wind.
