I was very fortunate to find a damaged SST Napoleon cannon on ebay, and the bonus was another damaged carriage and a separate barrel bought from the same seller. Here's a link to my thread on the repairs to the carriages, and the SST Napoleon:
CW 12 lb Napoleon Cannon - my Grail find and then some!
As lucky as this buy was, I don't hold out much hope for finding another SST cannon wheel, but there was a solution. I had bought a few one-sided resin wagon wheel wall ornaments from Hobby Lobby a couple of years ago They are molded to look badly weathered, but the size was close to the SST wheels, I found some "3M PATCH Plus Primer Spackling Compound" at the hardware store, and it was perfect for smoothing out the weathered wheel. It dried real fast, sanded smooth and could be painted right away (30 minutes) I started with one wheel that I cut into two pieces for battle damage. Then I had the idea to make another intact wheel. Part of an old wooden thread spool made for a better looking wheel hub, and they were drilled out for the axle. The damaged wheel was drilled out at a steep angle - it still supports the axle, but looks more damaged.
This is the original Hobby Lobby decoration with the damaged wheel. You can see the spackle filling in the rough parts, and some were left for a burned effect:
Here is the "battle damaged" Napoleon with the barrel dismounted. It will look even better with smoke and dead and wounded gunners laying around.
With the spare wheel, the cannon can be set up as a working piece. It will look great set back, in line with the actual SST Napoleon. The trunnions (pivot arms on the side of the barrel) don't fit on the carriage correctly, and the carriage is the Number 1 for lighter pieces, but in the background, it will look just fine