Your machine may be different, but in this video @~4:18 there are 2 pumps driven in tandem off the end of the engine. They are in the long black cylinder, center lower picture frame. The one on the right, with the larger oil feed hose, is the drive pump. The one on the left, with the smaller feed hose, is the cylinders and aux pump(usually). The size of the feed hoses and the lengths of the pump sections correspond to the oil flow rates of the 2 pumps. The longer drive pump has a higher flow rate and needs more power to do this. This is a typical set-up:Yes, my intention would be to replace only the motor and hydraulic pump. I'd select the new pump based on the ideal operating range (in rpm) of the new motor. I'd select a pump with the right displacement to give me the flow rate I want at that rpm. There is only a single pump, delivering flow to both the drive motors, lifting and tipping rams, as well as the auxiliary hydraulic circuit.
All right, we're getting close here. Now, envision your mini ex moving forward and back at travel speed as the boom, arm, and bucket are being operated. In reality, this would never happen as the machine operates in a semi-fixed position. But, this is how a skid steer operates.. Coincidentally, even in a modern mini ex (Kubota KX040 is what I have the most time in), I've never operated at full throttle. I only notice more speed over the ground when driving around, but no more digging power.