Of course, the kit includes only the mounting bracket, not the engine. Which shows that even back in the day, people thought of liability lawyers once in a while. That, or everyone just had half a dozen small motors lying around, in case a kid's project came along.
Check out the fuller, crazier wagon-tuning discussion at Hemmings.
Horizontal Shaft Engines FTW [blog.hemmings.com via dt reader dt]