Yep you can jump start a 6 volter with a 12 volter.
Just do NOT go battery to battery.
Make SURE that your tractor is in neutral.
Turn on the ignition switch.
Hook one side of the 12volt jumper cable to the big post on the starter.
Check again that you are in neutral. (We would not want to hear from your kin that the tractor ran over you!)
Touch the other clamp of the jumper cable to a good ground on the tractor -- i use a foot peg.
Polarity does not matter -- starter spins the same way either polarity.
Do NOT hit the starter button -- that will keep the 12 volts off the 6 volt parts on your tractor (battery/generator/regulator/lights/coil)
You do need enuf juice in your 6 volt battery to run the sparkies.
Roger in Michigan