Since no one else suggested it:
White plastic (leather imitation Ambrosio/Pelten) to keep the classic looks, but wipeable. Or blue cotton - untreated. For the cranks: If it hasn't got to be original why not convert a gold anodized "Campa style" (Campa, SR, ?) BMX crank to a double?
Tobit Linke, Dortmund, Germany