The S3X hub has what is basically a fixed "freehub body" that appears to be both splined and threaded. The splines and threads are perfectly clear in the picture of the hub...http://www.sturmey-archer.com/products/hubs/cid/3/id/47. The splines intersect the threads....nonetheless it is threaded. I have a splined-threaded BMX freehub that looks exactly the same, except, of course, its a freewheel, not fixed.

With the S3X hub, to ride fixed, it appears that all you need is a normal splined freewheel cog, which is held on by a normally threaded lockring that threads onto the fixed "freehub body" over the splines. At least thats how my BMX freehub works. The lockring threads are standard freewheel threads. From the looks of it, any basic (Shimano compatible) splined freehub cog should fit onto the S3X hub. If that is the case, it should be very easy to regear.

Alternatively, since the "freehub body" is fixed, you could convert it to freewheel by threading on a bmx freewheel. As I said, the threads should be the same.

Bear in mind, that I have never used this hub, so I could be wrong about compatibility, but I can't imagine that Sturmey Archer would use a completely proprietary spline pattern with non-standard thread sizes.