As far as displacement, 50cc is going to have real close to a 40mm bore and 40mm stroke.
The Photos section (linked above) has a couple Harleys.. one is red.
Also, it seems that AMF made a 'Harley" moped for a while but that one had a direct drive McCulloch engine over the rear wheel.. apparently sold through Sears. at least one is for sale including owners manual, etc..
There's not a lot of Google hits on Harley mopeds..