I have cats that live in the barn all winter and here, winter can be 40 below with horrendous windchills for days and even weeks at a time. They do have a 'house' which is a frame to suspend a heat lamp from and sides made of feed sacks. Bottom is raised and there is an old rug in there for them. They also burrow into hay and straw, and I suspect one or two sleeps on a horse. When temps drop close to freezing, I plug in the heat lamp (am using an old chciken brooder lamp) for them but other than the colour point semi-feral tom, most don't bother until it gets really cold. Some will sleep in the tractor cab until I close the windows, then they will move back to the barn, so cold is pretty much relative to what they get used to. I guess the short answer is to provide some kind of shelter for her, well bedded and insulated. For the record, all barn cats have all ears without deformation so they are keeping quite warm. They also get a diet of locally formulated barn cat food - high protein and fat as well as tinned food, and have a heated water dish which they drain regularly and they have developed a fondness for really warm water in winter.
Cats can and will get into engines, particularly engines with block heaters but a few thumps on the hood, and some horn honking before you turn over the engine gets them gone, and if you are concerned, do check first.
Outdoor cats do bury their 'business' if the soil is soft enough, but here, they just go to the manure pile and don't bother burying, or go in a stall, and wait for me to pick up the mess
Most cats don't have the luxury of a monster 10X12 litter box deeply bedded in straw
Forgot to add, MY cats rarely eat mice, at least some dont', but they line them up neatly in the tractor bucket and i just run them to the burn pit. At least they don't leave dead rodents around the yard for the most part - the old man trains them well; the old man is the oldest on the yard, neutered tom who did sire two litters and still hunts with his neutered sons .nd great gran