To remove the cog, either a slit applied by dremel as suggested earlier, or use a large vice and a pair of side cut pliers and a nail and a hammer. Mount the motor spindle gently between cog and bearing in the cutting jaws, sit the motor and pliers in the vice jaws so that the pliers lie across the jaws and the motor dangles between the jaws, but is not gripped by them. Then using the nail as a punch, tap on the end of the shaft. The motor will separate itself from the cog. Just a case of catching the motor when it does.
Both of these are a waste of tme for a radar, whatever voltage is used, it will spin far too fast. It does need a gearbox, handily provided by the servo makers between the motor and the output shaft. Leave the cog, remove the mechanical end stops (or the lump on the output cog) and remove the position sense pot.
The servo motor minus cog is ideal as power for small displacement boats, the main problem being getting drive from the stubby shaft. Flint springs fron dead disposable lighters work well for this.