Prop
Chuck Auger is steering you right. The Jett is very durable and parts are available.
But in the mean time try going from such a large diameter shallow pitch prop to a smaller diameter higher pitch. You are at the power end of the prop spectrum, rather than speed.
Consider a 9x9, 9x8, or a 8.75x9.25 Quickee 500 prop. With these props you should be able to tune the exhaust to the 16-17,500 rpm range.
Ray