There is no nut on the shaft but should be an easy find at a hardware store. Steering wheel center cap threads into steering shaft. The item "Rare Used Vintage Outboard Boat Red Steering Wheel & HelmWilcox Crittenden" is in sale since Monday, March 21, 2016. This item is in the category "eBay Motors\Parts & Accessories\Boat Parts\Controls & Steering". The seller is "horsehead_traders" and is located in Manistee, Michigan.
This item can be shipped to North, South, or Latin America, to all countries in Europe, to all countries in continental Asia, to Australia.