Being it is hydro start by looking for dead grass around the axle.
Your hydro pump is belt driven . Look for worn or glazed drive belt .
I see hydros fail from time to time from over heating from dead grass or the wheels contacting tree trunks or driveway block even tomb stones - impact shocks the system and makes seals fail.
I would guess you have a belt pump issue .
Your machine has a good history of staying out of our shop - common starter failures and engine valve lock nuts getting loose .
I have seen some burn up from dead grass being packed in around the motor ... Muffler is about 1200 degrees ...