If you don't spray down your engine it will eventually look like a turd if it isn't cared for. The 78's weren't painted and I have seen several that look good but I have one in the basement that looks like it is growing moss. It's that bad.
If I were you and wanted to keep it the way it is I would still paint it. But I would use Duplicolor clear engine enamel. Then it at least has something on it. But their aluminum color looks really good too.
I can't really tell whether my 78 was painted or just clear coated, but it looks like a GL500 where it's worn away. I'll take a closer look tomorrow, now you've got me curious.
I wonder what Honda used to clearcoat parts like the valve covers? Whatever it was, it's probably illegal to use now, funny how things become bad for the environment just about the same time the patent runs out...
Any BTDT with clearcoat on bare engine? Wonder which brands/types hold up best. There's also clear powder coat that might work well too.