^^^ also good advice...an air leak can make your engine a mother to start too. The cold shouldn't even be an issue once it starts. I really don't recommend this but on a really cold morning a quick shot of starting fluid will get it to fire right up on the first pull, just don't use too much of it, about a half second shot at the air filter inlet is all it takes. Don't sit there and spray it in for a few seconds tho... just a quick shot and trust its in there. If you gotta pedal and bump start it then you want to spray it with the choke open then close the choke and go for a start. You can shoot in another shot after a few start attempts, but the same thing... just about a half second at the air filter inlet is all it takes... the people that blow their engines using this stuff is a direct result from spraying too much in.
I'll also add this.... don't use it if you don't feel comfortable about it, and never shoot it into a running engine.
Common places for air leaks are at the carb to inlet connection, at the intake to the cylinder, the base gasket, and the crank seals. The case halves can also leak but not as common. If you suspect a leak you can use some Indian head gasket shellac and lightly coat the intake or base gasket, and a Thin coat of this on your inlet before sliding the carb on can seal off a small leak. A really thin coat of the gasket shellac or copper gasket spray on the head gasket never hurts, and the copper gasket spray can be applied to any gasket used on these engines and can be used in place of gasket shellac everywhere except on the intake manifold its self, the shellac will fill tiny voids where no gasket is used much better than the copper spray stuff. Both type sealers only need to be used sparingly to work, using too much wont seal a big leak but will make a mess.
If you find a crank seal leaking the only real fix is to replace the leaking seal. These usually leak on a new engine when the assembler was in too much of a hurry putting the engine together, luckily these are cheap and fairly easy to replace. Just mark your mag rotor so it doesn't get put back on backward or the engine will definitely not start since it'll fire at the wrong time. There are lots of posts about which way these go on as well and could be another reason yours don't start if it was installed backward at the factory... stuff like that does happen with these engines....
hope some of this helps.