My tips for cold start would include:
1) Remove oil bath cup
2) Shoot 2 seconds of ether up breather intake
3) Hold down clutch & turn over engine
4) Allow engine to warm up several minutes, then slowly release clutch
The colder it is, the lower your amperage output is from your battery. This will have a negative effect on your start-up. You can, in a pinch, jump the tractor (to starter if 6 volts) to increase the amount of power available to the coil.
You can also carefully use a hair dryer to warm your manifold. This will improve vaporization of the fuel.
If you use your tractor a lot in below freezing weather, I suggest using a lighter oil in both engine and diffy and getting a block heater or hose heater.
General good maintenance goes a long way to making any start-up easier, whether 6V or 12V. Keep your connections bright and shiny, your points and plugs in good shape.
The 2N I just finished fired up at 4F on the 2nd rotation and it's on 6 volts. I wouldn't hesitate to try starting that 2N engine at -20F.