With the old pump you have there you don't know what the GPM is without a flow meter.
It depends on what size pulley you'll drive the pump with. I always planned on using the PTO so I could turn the pump off when not needed.
I would run this one @ 1:1
http://www.northerntool.com/shop/tools/product_200329716_200329716
If it was to slow for running 4 cylinders (lift/dump) then I'd up it 1.5:1
If I used the old one you have there I would start with a 4:1 step down to see what kind of volume it might have , you could always change the ratio later.
I've got to re-state my 0.87 c.i. pump ... it was 0.087 cubes BUT I was at about a 3:1 step up on it. I drove it off the crank pulley on a Vega engine in a home-made tractor.
The main thing is not to use to large of pump as it'll pull down the engine and slam the cylinders.
A pump larger than the valve can flow will choke down the engine too.
To much pressure is
extremely dangerous so buy a pressure gauge and check your system at varying engine speeds before you start working the loader.
Remember you'll be working
probably 2" diameter bore cylinders so figure how much volume you'll need x4 cylinders @ normal running rpms. Then back off a little for safety.