The only reason this works on a sled is the transverse mounting of the engine and drive parts for the track. Once he's in the air, he hits the brakes hard and the entire driveline stops. The momentum is transferred to the frame and the front flip rotation starts.
For the past few weeks I've been playing around with rc trucks. When you drive that off a ramp for a backflip, you can stop rotation quite rapidly by tapping the brakes. The motor is not mounted transversely, but the oversized wheels have quite a bit of mass with nice momentum once going.
A jetski engine is not mounted transversely and jetskis don't have brakes.
Not gonna work.