Looks great! I have this same set and I love it. Like you, I had difficulty finding the circus stock car. Keep looking because I ultimately found one. Just like you, however, until I did I also substituted the MTH red and yellow 200 series stock car to which I added some G scale circus poster refrigerator magnets that I purchased on ebay.
In my mind, however, the train was missing something, i.e., circus wagons. So I purchased several 200 series flat cars to which I initially added some custom-made G scale, all wood circus wagons, which were designed to fit on G scale circus flat cars. I also purchased these on ebay. I could make them fit on the 200 series flat cars but really they were too large and only one would fit on a 200 series flat car at a time. I wanted at least two.
So I contacted the maker of the G scale circus wagons and over several months worked with him to design and fabricate some downsized 1/32 scale circus wagon and custom side rails. The latter were mounted on both sides of the 200 series flat cars. You can see the results of our efforts running on SGMA's Trainfest 2014 layout.
https://www.youtube.com/watch?...amp;feature=youtu.be
Come see these cars and several others in person at Trainfest 2015 in Milwaukee this coming November.
Bob Nelson