I have been looking at the wheels and hub kits from Grant Products. They offer a splined marine adapter for their wheels, as well as tapered shaft adapters. I believe that their hubs are a standard 5 bolt pattern, so wheels from most companies should fit the hub. I would drop them a line to verify that the hub is indeed 19 tooth.
http://www.grantproducts.com/c-67-Marine.aspxI was going to mentionthat you probably had a tapered shaft, like my 2007, but I took a quick look at the Four Winns parts catalogs for both years, and was surprised to find that sure enough the 2006 had a splined steering shaft, and the 2007 has a tapered and keyed shaft!