I think the example in post #28 looks the most "vintage".
These clone engines are just so ugly if left stock. The basic cylinder arrangement is OK {you're stuck with it}, but the identical squarish blobs on the rear {the muffler and the air cleaner assembly} should be changed out for alternatives, preferably metal.
Painting the whole engine block is key, cooling fins and all. White, copper, gold, or any of the old colors. Even some of the Evinrude and Johnson (outboard motors) paint colors would work. Paint whatever air cleaner assembly you end up with the same color as the block and same for the exhaust, with a heat resistant paint or just let it burn off.