We just needed a quick fix for an afternoon of wringing out the cart on some trial runs. We only had a couple of hours and I didn't want to take the time to run around shopping tubes, tires, or tire repair. I ended up just robbing a valve core from a bike tire and putting chewing gum inside a valve cap and screwing it on. It held for the few hours we needed it to. I'll get the tire fixed this week.
On another note, my boys have already decided that the 22-25 mph they are getting is fine for the moment, but they want more. Since we were considering changing tires, what's the best route to go. This cart will almost be used exclusively off-road or on gravel roads. Would changing the 18" tires for shorter ones like the front, and taking out the governor solve most of the problems, or should I keep the larger tires, change gear ratio? Would like to improve the acceleration, top end, and steering.