Align stock 5.1V regulator to drop 0.7V out of the 6V BEC is
M4N RL201 2A diode
You can buy a 3A diode 1N5401, which is slightly larger. 1N4001 is 1A version and smaller. It is good enough for a single digital servo like Futaba 9254.
I cannot find 2A version in my proximity.