You can still relatively safe use a 12 volt pack to jump start the tractor IFFFFFFFFFFFFF you jump just the STARTER ONLY, i.e. let the tractor battery (or Mag) power the ignition and the jump pack to power the starter motor ONLY direct.
OBSERVE SAME CORRECT POLARITY + to + and - to -
You can attach one jumper lead to the starter post/stud or else the TO Starter (NOT to battery) post/stud on a 2 terminal (Battery/Starter) external mechanical push to start switch (or 2 terminal solenoid) and then when you want the starter motor to crank the tractor, momentarily attach the other jumper lead to case/frame ground preferably near the starter somethewre NOT anywhere near the battery. If the ignition is on and you use the jumper to spin the starter she ought to start.
NOTE this isnt possible to do as I stated unelss you have an external 2 terminal mechanical push start switch or solenoid. If it has a one visible terminal saddle mount push switch you cant jump to its input post because thats same as the 6 volt battery with a 12 volt jump pack applied to it...
Just use the jump pack to spin the starter ONLY and let the Mag or Coil Distributor ignition be on to power the ignition
DO NOT use a 12 volt battery jumped direct to a 6 volt battery because the 6 volt can blow up !!!