S.B.
I would first wash the stroller padding out with a mixture of vinegar and water. This will cut the urine smell so the cat isn't so attracted to pottying there in the first place.
I wouldn't use the mothballs - they are toxic to pets and kids. I assume that you have little ones since you have a stroller. My neighbor puts them outside his house to keep the cats away and 2 of my kids have found these "marbles" and brought them home - not safe.
I know that cats do not like the smell of citrus and some pet stores sell kitty keep away sprays. I am not sure how effective these are but you can give them a try. For a cheaper experiment, you could try squeezing a couple of lemons or some lemon juice in a water bottle and spray the rain cover to see if it works (although if it gets rained on . . . I supposed your juice would wash away :(
Honestly, I would be working on getting rid of the cat rather than trying to deter it. I would call the neighbors and tell them what is going on or animal control. Loose cats are dangerous and destructive.
If you really wanted to get creative (AND spend a little money), you could buy a motion detection sprinker and set it out at night (I assume this is when it is happening). These detect motion and start up sprinkling water for 15 sec (or whatever you set it for), which is going to make the cat run away.
Good luck!