Your tractor came with a 3 post solenoid. They only make it in 6 volt versions but it will work using 12 volts. The problem comes in if you have a engine that has a no start problem. If you crank and crank the 6 volt will burn up from a constant 12 volts feeding it.
To get a 12 volt solenoid you have to move up to the 4 post version. You hook your orginal starter button wire to one of the small post. You will then need to add a new wire from the other small post to the key switch. This will also make it where the tractor will only crank with the key on.
Part numbers for the 12 volt 4 post relay.
Ford/NH and Tisco 311006
BWD S103 (places like auto zone or oriellys will sell this brand)
If the place you go does not have any of the above brands they can cross from these numbers.