Could be just about anything electrical. You will have to eliminate stuff like CDI unit and stator when it is cold. When you say someone told you it wasn't getting spark when cold did you see that for yourself or was that just a guess on someone's part?
It could also be as simple as not getting enough gas when cold. Those carbs have a very poor fuel enricher (choke). How many pumps on the primer do you give it when cold? I used to pump about 5 or 6 shots after I could feel it start to pump when cold and then leave the choke on until the idle slowed down.