Most times hydraulics are used the calculation works the other way , you work out how much work the pump has to do and you get a motor powerful enough to drive it , if it's not enough just get a bigger one !!If it turns out you need more work done just get a bigger pump and a more powerful engine to drive it .
You are coming from the opposite direction , you need work done and want to use the most efficent way of doing it , hydraulic pumps and engines are not very efficient .
Your idea to use the hydraulic pump will never be as efficient as a direct drive from the crank , it's takes power to drive a pump , there are friction loss's driving fluid through a hose , there are friction loss's rotating a pumps bearings and the friction loss's on the seals on the shaft and so on and so on , not to mention the same loss's on the hydraulic engine .
Come on Jibby , get a bit practical .