How much pressure does a power steering pump generate? Have you thought about using an old push mower engine? I see people selling refurbished ones all the time for $10 to $20. You might need to by a pump and a lovejoy connector though.
One of these days I want to convert a 20' 12K flat bed trailer into a dual use trailer. I was going to make the deck removable so it could be swapped with a dump box. While not free I figured I would use a 2 stage wood splitter pump connected to a 5hp lawnmower engine. The pump, tank, and box would be removed as one unit. A wood splitter pump would allow it to dump at a good speed for lighter loads, say firewood while kicking down to dump the occasional load of gravel.
I would calculate the pressure and flow desired for the cylinders you plan on using and then see if the pump can produce what you can live with and then if the engine will have the HP needed for the job. No point in making something that could be great handicapped.