The pump mounted behind the hydro must be small, about 3.25 inches square to fit, and it's a snug fit.
There are several pumps in this series with same dimensions. My pump was a .129 cu in. displacement, which provides 2 gpm at 3600 rpm. That's all I needed for my single cylinder, dumping the bed and gave a moderate cylinder operating speed.
You'll want a larger displacement pump, perhaps the one with .258 cu in displacement, giving 4 gpm at 3600 rpm.
The larger displacement pumps in this series are limited to 2500 rpm intermittent. Download the tech data from Northern's web site for the pump linked below, and you'll see the pumps in the series.
Here's a pump that might work for a loader--
https://www.northerntool.com/shop/to...9708_200329708
Four gpm might be a bit slow for a loader, depending on the size of the cylinders. This is a high pressure pump, 2300 psi continuous, so you could use smaller diameter cylinders, giving a bit more speed on operation.
As you design the loader, you'll need to take into account pump pressure, flow rate, cylinder dia and length, and lift and curl geometry. All those things determine functionality of the final design.
Remember that hydraulic cylinders often use very high force on a shorter moment arm to achieve greater movement at the end of the component. In other words, a cylinder that pushes at a point 1/3 the way down a beam, and lifts it 1 foot with 1000 lbs of force can lift 333 lbs, 3 feet high at the end of the beam. So the lever fulcrum location and length of the beam determine the forces needed to lift a given weight.
Archimedes said it all--"Give me a lever long enough and a place to stand and I can move the world."
If you decide 4 gpm is marginal in terms of capacity (as it might be) then you can use a larger pump and mount it differently--either further back behind the rear end, where a larger pump might fit, via a belt above the hydro unit's rear output, off the front PTO (as I suspect most are).
I'd try to contact the guy who built the loader just linked a couple posts back (where I got the idea to mount my pump) and ask him what pump he used and was he happy with the loader operating speed and capacity.
Be sure to look closely at the pump's rated RPMs as many are not rated at the Cub's engine speed--3600 rpm.
Lots of stuff to digest.