Yes, that is what you need. But it will more or less suck, as DS pointed out. And maybe it will fit on your bike and maybe not. Using the same setup on one of my bikes, I needed to use a pile of spacer washers, and spread the dropouts quite a bit. No prob on my steel bike, but it might be an issue with yours if it's alloy, and you need a couple cm more room.
What you really want is a new disk ready motor wheel. Motor only for a nice 9c motor is not that much money. Additionally, where you really need disk is the front wheel in most situations. Since it doesn't do much on the rear tire, v brakes may be adequate for your rear. Adequate if you run 48v that is. If you are in the 40 mph club, the screw on disk mount is simply not good enough, so spend your money on the motor if you are hauling ass.
Edit. Looking closer, now I see the wierd add on brake mount. Looks sketchy as hell to me. But it might make a nice torque arm. I was assuming your frame at least had the disk mount. Just another reason to just make the front brakes good, and use v brakes in the rear.