bpsteve,
Please correct me if I'm wrong but, I believe that the original Mt. Howitzer wheels were 38". So, being a half scale you would need 19" wheels. I know I've come across a few 18" & 20" wheels in my surfing. But, I don't think they were cannon wheels (14 spoke, 12 for Mt. Howitzer) but, more of a wagon wheel type. I'll see if I can find them again as it's been quite some time. Even finding a 19" wheel might be pretty tough as it would be a non standard item due to the size. However, if you come across a set let me know.