If I remember from my younger years, you could buy standard "pump drives" to mount direct to the bell housings of any motor. When I was looking at them you could get them in a number of configurations (2-5 pump mounts) and different size drives for each. They were realitively standard components and mount sizes.
Sparweb is correct, and I believe that skidsteers use this type of mount (many of which are 100 hp+). Bobcat, New Holland, John Deere.
Excavators also exclusively use hydraulics for power transmission. If you are looking for something used, I would start with these (the power unit is somewhat isolated).