Resurrecting an old thread...

From Sheldons site: "If you have 406 mm (20") wheels, you would need to install a 69 tooth chainring to be equivalent to a 52 tooth with common 622mm wheels."

FWIW, if you want to make your own chainrings, there's a template available on iHPVA's site that involves only using a drill press.

Here: http://www.ihpva.org/Builders/Sprocket/

Sizes up to (heavens-to-Murgatroid) 92-teeth from those templates.