It is very likely that ANY bike of that vintage will have trouble starting when it is real cold.
Bike design often doesn't anticipate that you will be riding when it is THAT cold.
Does it still have a stock airbox and filter ? How about pipes ??
Are you putting the choke on FULL and not touching the throttle until it fires ??
A dose of Berrymans B12 Chemtool in the gas might help clean out the low speed jets inside the carbs.
Don't use high octane gas unless the original design called for it.
Regular might fire up a bit better.