Tractors are hard to winterize satisfactorily for Serious Cold Winter areas. It would really be a whole big lot better if you could find somewhere to bring your chickens indoors for the winter -- maybe a corner of an outbuilding, a shed, a garage?
If you absolutely have to winterize a tractor, it can sort of be done but is generally not so nifty for you or for the chickens. You still need good ventilation (actually a tiny coop needs proportionately MORE ventilation, per square foot, than a walk-in size one) but because it's important not to have drafts (or blowing snow) coming AT the chickens, you want the ventilation to be not right adjacent to the roost and preferably coming from as sheltered an area as possible.
You can partially enclose the run part of the tractor with plastic or whatever, but, again, not totally.