I usually paint all my engines with a Duplicate-Color Engine Paint. Aluminum paint code 1615. They all sell a grey “filler primer” for use under that paint. I think it does an outstanding job, and is a VERY close match for the original Honda finish. I paint it all. It usually smokes and stinks a bit for the first couple of engine heat cycles, but after that, very durable and assuming the surface is prepped, very clean and the ambient temp is warm enough, no flacking or fading, in my experience. Oil resistant and cleans up well.
Having said all that, the ‘75 750F engine was painted by the previous owner. He used a “Eastwood Aluminum Engine Block Paint”. Again, very close match and holding up extremely well.......