tractor won't stay running.

Teasing the choke full out will open the valve enough to flood and provide fuel, though the moment you push it in, the fuel flow stops. As you mentioned you rebuilt the carb, perhaps you need to double check if you put that in upsidedown, the needle valve that rides on the float... You did not mention if the tractor ran fine before the rebuild, letting us know if there was no problem with the manifold or not. more info you give us the better.

They can crack allowing air (or exheaust into the intake) it also could be gasket problems at the head.

use a propane torch with the head taken off so it is just the pipe. (do this outside with care and no gasoline splashed about)
turn on the propane moderately (like you were using the torch) and direct the gas around the outsaide of the manifold while someone cranks the engine. If it fires up, you found the leak.

If not it may be internal Looking down the exhaust pipe (or with the pipe removed can find a crack)

good luck, let us know what the cure is
JimN