"I did see some wording that said, use with external resistor"
If it says that my best "guess" is it means just what it says i.e. it requires the resistor or else it will overheat....Id try that coil WITH the unshunted external ballast n see what happens.
"Since the tractor has 12 volt system should I ask for a 6 volt or 12 volt coil to install on it?"
You can EITHER use a 6 volt coil PLUS the external series voltage dropping (12 to 6) working ballast orrrrrrrrrrrr a 12 volt coil with no ballast required. However to utilize the ballast by pass system for improved cold starting you would need the 6 volt coil plus a good ballast. Id be tempted to just use a new full 12 volt coil (NO ballast required) and do away with any ballast or by pass circuit headaches...