Donar Dilemma

[Bonzo]

Personaly I think it will be more straight forward to complete the build from your single Sierra donor, provided your paperwork is in order, you should qualify for an age related plate.

In theory, you can build with a different engine & still qualify for an age related plate.
That said, you do risk opening up a can of worms as some local DVLA offices may have a different view & slap you with a Q plate.

If you use both engine & Gearbox prior to IVA you'll almost certainly be given a Q plate.

You have not stated the age of your Omega, it could be entirely possible that you may need to fit a CAT to meet the emission requirement's for your engines age !!??