You didn"t say whether the problem was slow starter speeds or whether you noticed some else when you attempt a cold start. I live in Western Montana and have a 12V TO-30 that starts on the first turn, independent of the temperature. Your 6V system at best is marginal in really cold temps so the ignition, fuel and starter must be in excellent tune to reliably start in coldd weather. Here are some thing for you to look into.
Are you keeping the clutch pedal down when you try a start? Most of us leave our pto/hitch connected so if you are not disconnecting the transmission and pump by stepping on the clutch, your starter is trying to turn a cold engine AND the cold tranny and pump.
Carbs are not temperature biased so they need a choke for mixture enrichment in cold weather and they generally need the main jet opened up a little for cold weather operation.
Your battery needs to in top shape and you need to have big starter cables especially for a 6V system. The chasis grounds must also be good.
Make sure that your ignition system is up to snuff giving your the requisite FAT, BLUISH-WHITE SPARK, the color of lightning. Make sure that you timing is correct and that your advance is not sticking.
Good Luck, from a fellow Hoosier.