I guess it depends what you want your engine to look like; rebuilt or not rebuilt. Diana's engine is rebuilt - it's easy to tell as everything is black. 4BD1s pre-rebuild have bare aluminium thermostat covers, inlet manifolds, injection pumps, fuel filter brackets, heater pipes, fuel pipes, exhaust heat shield etc.
I think the rebuild process must have included a step to spray absolutely everything gloss black, the texture of what was sprayed determined it's lustre.
Then again, not all army engines were just black either

(This photo also shows what a non-rebuilt 4BD1 looks like if you imagine a black acoustic cover)
