I don't profess to be an expert on this matter but if I were to try to reverse engineer a solution I would start out with the hydraulic power available on various units--which are designed for varying size of mower. The 422/25 series is approximately a 4' mower. The 1430 is approximately a 5' mower. The 1845 runs a 6' mower---etc. You indicated you are thinking of a 5' mower so I would use the hydraulic power for the 1430 and identify a pump that could be installed on your unit that will produce the
PTO hydraulic power of the 1430.
Most mowers in this size range use at least 2 blades and 6' mowers typically use 3 blades so direct drive is not a practical option--rather belt drives are used and that introduces the option to run the blades at a speed other than the motor speed. Selection of blade speed is a function of the size of the blade because blade tip speed is the design criteria. Once you know the desired tip speed, the effective gearing between the hydraulic motor and the blade, and the hydraulic flow you can select the displacement of the hydraulic motor. The motor selected will need to be capable of handling the max pressure available.