First of all, typical "old farm tractor" ignition coils DO NOT HAVE AN INTERNAL STAND ALONE DISCRETE "RESISTOR" TUCKED AWAY HIDDEN SOMEWHERE INSIDE THE CAN. A few old auto coils DID HAVE an actual internal discrete stand alone "resistor" inside the can as the pictures illustrate.
Many full true "typical old farm tractor" 12 volt rated coils have around 2.5 to near 4 ohms (many 3) LV primary winding resistance as measured between its small + and - terminals and that's made up of the wire ONLY and NOT a stand alone discrete "resistor" inside there.
Many typical 6 volt rated farm tractor coils have around 1.2 to 2 ohms (many 1.5) LV primary winding resistance, again NO INTERNAL RESISTOR
Many 12 volt tractors use a 6 volt coil PLUS an external series voltage dropping (12 down to 6) stand alone ballast resistor, be it a bathtub style or resistive wire link or a thermistor type device.
A coil labeled "12 Volts" or "12 Volts NOT for use with ballast resistor" is a full true 12 volt coil HOWEVER BEWARE if labeled "12 Volts for use with or requires ballast resistor" its in reality more like a 6 volt coil and needs the ballast JUST LIKE IT SAYS !!!!!!!!!!
NOTE as this is an old tractor forum and NOT an old car forum I'm talking about "typical old farm tractor coils" NOT old car coils other then the reference above. Also not knowing if your tractor is original or has been modified or how over the years I purposely made the above generic in nature.
THE PROBLEM YOU DESCRIBE COULD POSSIBLY BE CAUSED BY A WEAK SPARK, THE CAUSE OF WHICH MAY BE AS NOTED BELOW AND/OR THE TIMING ISNT CORRECT Have you checked the ignition timing lately ?????????????????
A) Burned or carboned or incorrect gapped points...
B) A weak or bad or incorrect sized condenser...
C) Bad plug wires or a cap or rotor problem
D) A weak ignition coil
E) Fouled spark plugs
NOTE often a weak condenser or a weak coil will act okay until things warm up before it starts to fail !!!!!
NOTE are you sure the coil is wired to match the tractors polarity????????? as if not the spark is weaker !!!!!!
NOTE even if the plugs are new they could be carbon or oil or gas fouled which can be another cause of hard to start !!!
Sure there may be fuel or carb related issues but you need a good strong blue spark (even at full temperature) and NOT any faint thin wimpy yellow one lol
Insure the coil is the correct one and voltage,,,,,,,,,,insure coil is wired correct,,,,,,,,,,,check for a good strong highly visible BLUE spark,,,,,,,,,,try another condenser if you have one,,,,,,,,,,check points gap and ignition timing ........of course the coil may be faulty or its not the correct one or gets bad ONLY after she warms up IE could be a coil problem !!!
Best wishes, hope this helps
John T Retired Electrical Engineer