With the seat and fenders off, I'd verify that the rear end of the hydro's top shaft (over the differential, sticking out the back of the hydro) is turning the same as the input shaft. The connection to the input shaft is pinned to that shaft. Sometimes it's just a sheared pin that fails to transmit power from the drive shaft into the hydro itself. Other times it's internal to the hydro. but a sudden failure like that points to a sheared pin.
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