Mate! your ski is ancient and not worth spending much money on. But if you really want to, it will be drive shafts if it made a grinding noise when it slowed up. Or it could be drive coupler. Something in the drive line. Its a pretty easy job pulling apart on those old things if all the bolts arent seized. If its drive shafts, it will probably be the short shaft in the engine bay thats stripped due to the pump housing bearings failing and washing the grease from the spline. Or it could be because an engine mount has failed.