It's correct that O.S. no longer produces a reverse crankshaft or camshaft for their engines. Demand has just become much too low. Any engine that has a one-piece crankcase cannot have its operating direction reversed without a reverse crank, four-strokes need a reverse camshaft. Older O.S. engines (or any other brand, for that matter) that have a removable front housing can have their rotation reversed by turning the front housing so that the carburetor points in the same direction as the exhaust port, or towards the right-hand side of the engine.
Some Saito engines can be made to run clockwise by fitting the camshaft from the opposing cylinder of the twin engine that resulted from combining two of the singles. So, while Saito does not produce reverse camshaft for the express purpose of reversing a single, it can possibly be done.