Even if it is possible to renew just the motor, you would still have the possible problems of damaged gears and leaky seals. Better to renew the whole unit.
Hi you are better off replacing the entire unit not to difficult, not to expensive, but make sure you buy a new rubber grommet as that deteriorates with age and give it a liberal coating of silicone grease when refitting grommet they tend to last longer, thats probably where your leak is.
Steve.