Cold weather amp draw from the starter is my "first cause" place to look. Voltage at the coil must be above 4.8 v while cranking to make good cold weather "fire" at the plugs (assumes 6v). A list:
Measure voltage at the coil while cranking.
if lower than 4.8, check the voltage across the battery (posts not cable terminals) while cranking. If lower, you might need a battery.
Double those numbers if 12v.
Attempt to start it by using the tractors battery only for the ignition.
Use a jumper cable (2 gauge real jumpers, not "booster" cables), and attach the ground to one of the starter bolts, and touch (in neutral clutch blocked down to reduce trans oil drag) the copper link between the starter relay and the starter motor terminal. This allows only full tractor battery to get to the ignition, and jumper amps to crank it.
Over choking can flood the engine and flooded engines quench the spark by having liquid fuel between the plug electrodes. Gasoline does not ignite as a liquid. Older plugs can sop up gasoline easier than shiny new ones.
Every battery cable should be 00gauge and as short as possible. Running a ground cable from the battery to the starter bolt is another voltage increaser. Any smaller and the distance to the starter will drop voltage radically.
Modern 10-30 oil is another speed picker upper for engine cranking.
They are pretty easy starting engines. Give some of these ideas a try. Jim
If it is still 6v I suggest a transition to 12v.