The Doncaster factory made the smaller four cylinder tractors which still used the 1970's chassis and back axles. Everything totally outdated years before it closed. Even the French built Maxxum tractors and transmissions, which were a late 1980's design, initially meant to replace the bigger David Browns in the Case lineup with transmissions manufactured at StDizier were becoming obsolete by the end. The StDizier plant was sold to a Korean company who continued to make the powershift transmissions for some years before closing it down.
What if Mc used a Cummins? It would be almost identical to the current line-up because the Iveco/FPT NEF engines were co-developed with Cummins but Cummins pulled out to make their own version which is slightly different, for and in N America. The Iveco engine doesn't have far to travel within Italy between the Iveco factory and ARGO. It has a lot further to travel North to New Holland at Basildon, between London and Southend.
The Doncaster IH stuff was outdated yes but I don’t believe the Maxxum line was. The Fiat take over was about 1999 wasn’t it? The ‘New Maxxum’ had a new cab and styling for just two years, the mechanics were 10-12 years old.
I’m sure if you had a look at MF or NH their offerings were every bit as old, particularly the Coventry designs of MF and Fiat derived mechanicals in NH.