Hello!
As somebody with experience with soviet bloc motors I can tall you that most of these motors were not painted at all - I mean the cast parts, like the block and so on they were left in natural metal, because the gaskets leaked and everything would soon be covered in mixture of oil and dirt - and this is what you want to recreate here. I'd suggest painting everything flat medium gray and then going over that with more than one black wash, preferably somewhat glossy to show still wet oil in the recesses and more dry dirt on the exposed parts. Stamped metal parts might be painted gloss black when new, but that would quickly get dirty too.
Hope this helps - good luck with your build and have a nice day
Paweł