Re: Oil tank assembly
The old switch is most likely fine.
Often the tank comes as an assy, including the magnetic switch that slides up into a recess in the bottom of the tank and is retained by a screw.
I take it you are replacing the tank because of a constant oil alarm any time the key is On, even before starting the motor, caused by an adhesive failure allowing the magnet on the bottom of the float to fall off and trip the magnetic switch.