I was in the exact same situation you are in. I inherited a tractor mounted log splitter but no tractor. It had been put on wheels and set at a height that was comfortable for me to work at, but it lacked a motor, pump and hydraulic tank.
Since I am a teacher, I approached our VoAg instructor to see if any of his students needed a "project". I had a 6.5hp motor from an old lawn mower so all I had to purchase was a pump and a filter from Northerntool. The class cut and bent sheet metal to form a tank and welded fittings into the tank. When it was done, I had spent about $175 dollars at Northerntool and about $50 for the materials the shop class used and 2 dozen donuts for the kids.
Now I am comtemplating having them do a log lift and catch tables for the end.
I will try to take some pictures this weekend if you would like to see them.