What was the last single-cylinder motorcycle Harley-Davidson made for production?

That depends a lot on your definition of "made for production". The last single made in the US by Harley was the 1922 CD but the very last singles to wear the Harley-Davidson name were the 1978 Aermacchi SS-250 and SST-250 models, made in Italy but with the Words "AMF Harley-Davidson" on the tank.