For 16 chickens, you should be looking for a minimum of 64 square feet inside the coop. If you can provide some details on your climate and location, you will get much better advice regarding ventilation and other factors to consider. In looking at the Google image of the coop you referenced, the enclosed coop portion looks like it will need to be tripled or more. Can you provide some measurements for reference? It might be easier and more cost effective to just start over. If that’s not an option, you could start by thinking about enlarging and converting the run portion of your current setup into coop space by adding siding, a roof, access door, and ventilation. If you can provide photos of the inside and outside as it currently it built, that would also be helpful to get more specific recommendations.