Over the years I've had 3 diff ATV out in the worst northern Minnesota has. Temps have been down to -20 degrees overnight and the key is a warm battery for sure. All I do is take the battery into the cabin with me at night since it's a long walk out for me. They have always started.
I insist on having a pull start, I have stayed away from fuel injected. The smaller the engine the easier they start I think. Current model I have is a twin 700 with
pull start...like I say the key I think. When it's real cold I only use the pull start, it just seems to fire better.