You could build a wire wall and split the area. Keep the end with the lawnmower as storage and feed cans. With all that green, you have plenty of free material to cover the concrete with to start the deep litter. Dried grass clippings, garden waste, leaves, pine needles, compost...
Fix the 2 hole "nesting box", add a roost on the back wall, throw in 4 pullets and you are in business.
You will soon wonder why it took you 2 years to get chickens. They are such a compliment to gardening and are more than willing helpers