I wont ask why you cannot just go out and buy a motor, but I will say that you should do it.
It is much easier to buy a motor that has a torque output that matches your requirements than it would be to design one.
To design a rotor form that gives the torque and displacement you require would be tricky to say the least.
The distributor valve design and manufacture would be equally tricky.
You could spend thousands and take several years to design the perfect motor to suit your application, then find one with exactly the same performance at your local motor stockist.
All of the above excludes work on castings, shafts and shaft seals...etc.
Adrian