Slim, I took over the management of a nursing home years ago. On Wednesday at noon I heard a lot engine noise and learned that the generator came on every week at noon. It sounded like a damn race motor. I went in to see what the racket was and there was a 440 Chrysler in the utility room. It ran on natural gas and did not have an air filter and had cracked exhausts and leaking pipes. I decided that was going to be problems for numerous reasons and bought a diesel system and mounted it outside. I have to admit that a 300 hp generator never had any trouble keeping up with our power needs.
If you are insistent in mounting the unit in the garage, I would ditto comments above about ventilation and ducting. All of my generators have a flap on the exhaust that pops open with use, but shuts when needed to keep nature out of there. I would also suggest a wire mesh screen for your fresh air intake to make sure it comes in clean all the time too. You will want both up high enough to make sure the snow/ice will not block them. Look for water heater or heater exhaust pipe that is double insulated, or pipe from a fireplace store. I would also consider some sort of cover and framework maybe made of 2 x 4 and some foam insulated board. You could use real duct tape (the foil and adhesive kind) to hold it together. I think the CO2 detector is crucial too, mount it on a far wall so it doesn't get any false positives. Since you have your toys in the garage, I suggest both a heat and smoke detector.
