As far as I'm aware the part number isn't the specific issue, you need to find the correct two letter code, be it 'BX' or whatever, and most crucially, what vehicle did it come off? Because it's a 'brain' it knows what vehicle it's from. Handily, the only info it 'knows' is engine and gearbox.
ie: 1997 2.5 V6 manual saloon in green ECU will fit a 2000 2.5 V6 manual estate in silver. Engine and 'box are only bits you need worry about.
wheras two silver Omegas, both saloons, both 2002 reg, both Elite, both got leather, registered on the same day, but one's a TD and one's a V6 it's a no-go.
Had this on a friend's car once and you will probably find you get lucky and get one reasonably quickly. There's not too many engines on the Omega, and most are Autos etc..
Just to add another important bit, you need to get one off a car with the 'correct' TC. If memory serves pre '97 cars are single channel Traction control, post '97 it's dual channel. So probably any pre-97 ECU is no good to you. Is your car definitely a mini-facelift? (Soft touch buttons, clear headlamps, lighter interior colour schemes, etc)