Hi guys.. this is my first attempt at a contribution.. so I proceed with caution and suggest that you take it as such.
In my early years, I served a number to tours as a chevy mechanic, with some decicated time in the truck shop working with PTO on agricultural applications. PTOs came in a seemingl endless variety depending on the application. But in general, the inside workings were generally farily simple. Certainly gearing, spockets and chains can be easily inspected. Bearings, chains and seals can often be found if you can get the numbers off them. Shafts, sprokets and gears are another story though.
So if you have one that is leaky, it's very possible off the shelf solutions are available. If the bearings are galled, they may also be avaialble. I think you can do a inspection without doing much harm if you're careful. Reember.. clean is mean

For what it's worth.. if you can seal around shafts and not use a pressure washer or air directly on those aread, you should be able to put it in a parts washer without hurting the inside.
I sure wouldn't reinstall it if it's not sound...no sense in asking for trouble.