I have a ZD331 with the 60 inch. 1800 hours on it. The lower pto gear box seal went out and we didnt catch it, so the bearings went out. I replaced the bearings ( the bearings are not welded to the gearbox) and seals, which lasted about another 200 hours and it failed again. So I bought a new box from dealer for less than $1000. I also have a ZD21 with over 2000 hours and have had no trouble with it.
The gear box failure I take full responsibility, for not catching the leaking seal. I suggest you do the same for an engine failure.