Modern power plants could extend old hulls, at a price

Could modern power plants keep World War II battleships operational or in reserve?

Technically yes, with much better power density and lower crewing, but the conversion would be complex and costly.

The answer separates technical possibility from an affordable programme. Modern machinery offers much greater power density than a World War II plant and needs fewer crew. Those advantages could help extend an old hull's useful life. The conversion would still be difficult and expensive, so modern machinery is not presented as an easy route to preserving a battleship.