One niced thing about single cylinder engines is the ease of modifying them. You don't have to match four (or or eight or twelve) ports to get good or maximum power. This has led to some interesting applications with the Rotax engines I get to work with. Ron Wood has used a dual exhaust on his dirt track singles with two different length headpipes, which on the dyno gave added horsepower and a broader powerband.
A customer in Sweden, racing the new DOHC Rotax 605, discovered that staggering the lengths of the intake manifold also led to a broader powerband, as well as making it easier to get the carburetors packed closer together. Something else that works well is BIG intake ports. A 44 to 48mm carb is run on the single cam engine, and the DOHC runs TWO 40mm carbs on a 600 cc single. These big ports may eliminate some of the drag simply by moving the stagnant air farther away from the wall. The carburetor people still try to tell me that a 38-40mm flatslide carb will flow more air at smaller throttle openings, but these are racing engines and are usually run wide open. I don't see how you can get the same volume of air to flow at the same rate at the same pressure through two different size holes!