Actually, if you switch the system over to 12 volts then you need to drop the voltage down at the coil to 6 volts when it’s running or the contact points probably won’t last long.
Many 12 volts from the factory vehicles drop the voltage to 6 after engine startup with 12 volts being provided only during starting.
This is usually done with an external resistor or as in the case of the old 12 volt VW air cools; the resistor was internal to the coil.
The tractor ran fine out of the factory when new and made it all of these years on 6 volts so the voltage likely has nothing to do with the problem. Still curious about if the carburetor was soaked during the overhaul though.
I’m with OK and MGMcAnick on this one.
Soak the carb in a bath of real carb cleaner, (not your gumout spray can stuff), and make sure the fuel from the tank flows freely down to the carburetor.
On a slightly related note, 40 years ago I rebuilt an engine on a '54 Ford 4-cyl tractor (it was seized). After putting it back together, I spent two days trying to start it, without any luck. It would run for a second with a spray of ether, but that was all.
I called a local Ford tractor dealer to ask for any input. They told me I had to remove the spark plugs, squirt a bit of oil into each cylinder, and try again. They said they had to do that with all the new tractors (off the truck from the factory), otherwise they wouldn’t start.
The oil was needed to “seal the rings” to increase the vacuum for the updraft carburetor to work correctly.
One suggestion. Might want to check your rotor button. That rotor, if its like mine has to have a clip on the distributor shaft for it to set properly. If its not on there, you can probably turn that rotor about an inch back and forth and it be be fireing out of time. But its pure luck if it runs anywhere close to normal even with the choke on.