Well I have the sohc style bike so it is a bit different but air cooled motors are cold blooded to some degree.
that said, tuning makes a big difference. In cold weather, a common trick is to change to a hotter plug. For instance, my 78 uses ngk D8EA plugs. Once things drop and do not really come above 40 or so, I switch to a D7EA. This is a hotter plug that works better in colder temps, just remember to switch back when temps come back up to 60 or so.
I have ridden my bike below 32 degrees. I pull the choke, slight twist on gas and release and then normal starting procedure. Bike might crank for a couple seconds and off it goes. Idle for a bit to warm up some and all is good.
So just try putting a the next step hotter plug in and see how that goes.
just fyi, people ride in snow and such on bikes like these.
Dave, how short is your exhaust? I have heard of this but it was on exhaust that barely came to the bottom of the engine.
The ride IS the adventure. The destination is just to get gas!