I believe I would get a parts manual from Terry so you get a hyd motor that matches up with the other hyd motors. He should be able to Email you some data.
If you use a different hyd motor, say a 25 cu in, or not the same c n displacement, then the rpm for that motor will be different that the rest, with the same GPM.
What is the manufacturer and numbers on your current hyd motors?
Can you verify whether the motors are in a series/parrallel arrangement?
Did you ever contact Terry. He has been most helpful to all of us.