My husband is OBSESSED with buying one of these things and modifying it to go this fast for our daughter. I'm delighted and terrified by the idea of it.
I wonder how much work it is to modify one of these - those look like regular old 2-stroke engines, like you'd find on a weed whacker or leaf blower. It's clearly just hanging off the back of the jeep.
You could probably do this in a couple afternoons. Mounting the engine is probably pretty straightforward. Hardest part is probably adapting the drivetrain to the gas engine. I'd imagine you might want to swap off the plastic bogey wheels that it came with for the wheels off a riding lawnmower, though...
UPDATE: Never mind, the creators pretty much said these are gas-powered go-kart chassis (chasses? chassises?) with a Power Wheels shell on top. That's not nearly as fun.