The Harken cars used to come on a bit of plastic that was the same shape as the track, it had an indent in it and that allowed you to reload new balls pretty easily.

So, you could get a short section of the same track you are using today, take a round file and make an indent and use that to reload the balls.

the indent needs to go on the bottom of the track, so when you take the car off, you can flip it over and drop the balls into the groove on the bottom