You can leave the sprocket on but taking it off will lighten the load a bit. Too, taking it off will give you a ring of open holes, one of which will work good to put a lifting ring into as you pull it.
In some cases you can pull the motor off the final drive, but typically the way they are made the motor's front housing is basically the rear of te final drive housing. In other words the two parts are integral with each other so there is no way to remove the motor alone/complete from the assembly.
I realize there are dozens of different types out there, and I haven't messed specifically with a Hitachi before, but in general what I have described so far is what you should be seeing, or very similar to it.
Below is a link to the place I called about the one for the 331 Bobcat. The guy I taked to was very nice, and as mentioned previously, honest enough to tell me that I'd be better off to get one from the OEM in my case. Never done business with them before, but to me honesty from a salesman goes a long way. Hope they can help you out if the need arises.