Looking at the list of parts, there seems to be two choices, for the retainer and insert, but no indication of which goes with which pump.
Some even cross reference to later Astras for superceded parts...
Have swapped a handful of Omega pumps, and other than the fiddly part of unclipping the pump assembly from the basket, have always gone to plan. Sounds like that was pure luck
You know the running issue is the pump itself, so would be churlish to scrap it for the sake of a single, available part.
An unused new part will still have value, so it's not a total bust.
Also, and I know that it is used, so will have a limited life, this
https://www.ebay.co.uk/itm/FUEL-PUMP-assembly-Omega-B-2-0-2-5-3-0-estate-saloon-9119098-93181359-Petrol-/113807236104 could be useful...
If you're lucky, the new pump will fit, if not it buys you time to get the correct pump for the existing casing.
Either way, this will drop straight in.