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]
Seriously, WHAT could possibly go wrong?
The motor isn't jacketed in any way, I'm sure there are no covers on the pulleys and belts... Oh, and it's a friggin' WAGON. That generally means no suspension, brakes or other 'safety equipment'.
After personally crashing wagons (unpowered), bikes, and motorcycles, I cannot imagine the carnage a 5yr old could cause (and suffer) in a gas powered wagon.
I'd say go electric but the last thing it needs is more torque.
"Learn many lessons they can use later". LOL.