Again, I think almost none of us can speak with certainty for Euro-spec cars. There may well have been, but in the UK there wasn't, unless there were some special order - perhaps government or Royal cars had 'special' underneath parts. (yes, the British Government and Royal Family's official cars were Omegas for a while, plus unmarked / CID Police used them too of course
)
As has been advised, get yourself a good shell, then engine, the rest is changeable. Ok, now there will be 'Opel Omegas' with LSDs - what we call Carltons (GSi 3000) as well as Senators. You're probably aware that they were named differently in the UK to Europe, but just wanted to clarify.
It is possibly to install the LSD internals into an Omega diff, though the other thing to remember is that much of the 'limited slip' of the diff has worn out on higher mileage. I posted a link some time ago (get searching on here!
) which a very well known engineering firm tested one after about 80k miles, and found a large proportion of the 'limited slip' had worn out. Its just the design of it, it's like a clutch, they wear / wear out eventually. I wonder if fitting another LSD from another car would be doable, rather than a lot of time and expense in refurbishing an old Omega diff. But that's something for you to think about when you#ve got the thing up your drive! No point getting bogged down in all that yet