Probably not going to like my answer, but grounding plugs are never used in the restoration of these fans, even the ones sold publicly by members of the AFCA that professionally restore them, and by specialty companies like VintageFans LLC.
When it comes to history, the NEC can cram it and take a hike, IMHO.
A polarized plug is plenty of protection for these fans.
Note 2-wire cord
Mind telling me what fan you have?