Unlike some later models, I doubt that tractor was originally equipped with any ballast resistor and ballast by pass system (hey a guy could add it) which did improve especially cold weather starting. If its a 12 volt tractor and a correct full true 12 volt tractor, a B isn't that hard to crank over, and should start fine with 12 volts and a 12 volt coil. IS THE COIL WIRED AT CORRECT POLARITY??????????? For + Ground as original, coils + wires to distributor.
HOWEVER IF IT DOESNT the cause may be:
1) Starter is dragging battery voltage down too low, which could be caused by a weak battery (have a shop test and load test it) ,,,,,,,,,,,bad loose burned battery or starter or ground cables (Remove, clean and wire brush and reattach each and every battery and starter and ground cable connections),,,,,,,,,,,,too small cables,,,,,,,,,,,,,,,Ground may be to a weak wimpy rusted sheet metal component like a battery box (try a good frame ground connection or even run ground cable down to starter) ,,,,,,,,,,,Starter motor is defective and draws excess current.
2) Defective resistive ignition switch contacts reducing coil voltage. Try hot wiring the coil direct to hot ungrounded battery voltage source and see if it improves.
If you put a voltmeter on the coils input with ign on and points closed, it should read near battery voltage. If its like .5 to 1 or more volts less, ign switch or wiring may be bad/resistive. If you put a voltmeter on the coils input and crank her over it may drop to 10 or 11 volts (depends on engine and compression and temperature and battery and starter condition) BUT IF IT DROP MUCH BELOW 10 VOLTS THE BATTERY MAY BE WEAK OR CABLES OR CONNECTIONS OR GROUNDS ARE BAD. I like at least 14 gauge wire to and from ign switch and down to coil to reduce voltage drop.
I suspect low coil voltage while cranking perhaps from a bad resistive ign switch or a weak battery or too small cables or bad cables or connections. It don't hurt if the points are good and clean and gapped correct and plugs and plug wires are good. How do the plugs look???? If you want to use a 6 volt coil and ballast and ballast by pass that can help, but Id check the above first.
Sure there's more possibilities, but chew on this a while
John T