It's true that the Crossley engines in the Co-Bo were 2 stroke and that they had, to say the least, weaknesses. But the GM engines in the 66 locos are 2 stroke as well. They are very good indeed and the design has antecedents that go back well into the forties (some might say the thirties).
If BR had instituted a program for the development of a 2 stroke engine in the late forties/early fifties there would have been plenty of time to iron the bugs out, but instead Riddles was appointed CME and he loathed diesels and so developed a totally unnecessary range of steam engines, purely as a vanity project.
Don't get me wrong here, my preference is for steam, very much so, but the steam engine was obsolete by the time of nationalisation and it was time for a change, not more of the same.
I don't think that 2-stroke,
per se, is a key aspect. In the USA of the major builders, GM and Fairbanks-Morse did 2-stroke (which the USA calls '2-cycle'), and Alco, later GE, and Baldwin did 4-stroke. The 2-cycle machines had their pinnacle in US Navy WW2 submarines, where weight is important but they effectively run either on or off. Fairbanks Morse units were also vertical opposed piston, so part way to a Deltic engine, just two crankshafts instead of three.
GM had bought the Winton company, and their 2-stroke expertise, in 1930, they had been the engines in the surprisingly numerous USA "Doodlebug" railcars of the 1920s, and apart from marine installations this took them on to the classic high speed Zephyr diesel trains of the 1930s, and others. One manager at GM, Charles Kettering, was the inspiration behind 2-cycle, he drove the purchase of Winton, and moved on to comparable bus and truck engines, which dominated USA buses from every manufacturer for the next 50 years.
In any event, GM's expertise came from exceptional sales and marketing, including extensive financing at a time when this was unusual, and also after sales support of spares, training, etc, again as unknown anywhere else. Compare this to UK diesel manufacturers who were not unknown, in times before formal support agreements, to say to buyers returning for spares literally months after purchase that the unit was now regarded as obsolete and no spares were available.