hmmm...not terribly cold...Well, I'm in NY, and it can get into the -teens here. I'd stick with the air filter that came with it, most restrictive part is the head, not the air filter. Exhaust, measure whatever it has, I don't remember what size is on mine, I just replaced it with stock parts a year ago.
As for reliability, these are incredibly durable engines. Mine currently has just shy of 200,000 miles, and Citoriplus (where is he, btw??) had over 300,000 on his, there is another member here who hit 500K, etc. They last a long time if you take care of them. They are NOT the easiest engine to work on, especially in their last form (2.5)... Too much extra crap added on.
No power steering...keep it that way. Personally I prefer manual steering, even in the dumptruck at the farm.