When they were built it was a toss-up as to whether a Mirlees or Ruston power unit would be used. I presume there's no chance of getting replacement Mirlees units, but does anyone know how easy it would be to retrofit a Ruston?
I do not think that an engine replacement is on the cards because for specific fuel consumption, simplicity and "grandfather rights" for emissions the Mirleees remains desirable. But let's imagine that it was. There might be three competing philosophies you could chose from.
One is to remain - as you suggest - with the "large lump" school of thought from a different engine brand. Perhaps the "straight eight" Mirlees unit could best be replaced with a custom V8 GEVO or EMD1010 from the states heavy freight scene. Similar power, but probably that would be the problem, these units would be no great step up from the existing unit in power or fuel efficiency, but would be better in emissions.
Another option could be a faster revving engine like the MTU 4000 or Cat 175 family. This could be a good way of getting more power whilst remaining in the same axle load (assuming the traction motors can be fettled to take the power).
On the other hand it might be better to pick some sort of genset system. This might be the best bet overall because the (previously specialised heavy haul) loco would become more versatile.