The bearing isn't usable with a ball missing. We need the dimensions of the bearing: the inside (or shaft) diameter, the outside diameter, and the width. If your bearing is the same as on the Model 51, those dimensions will be 17x47x22.2 mm. If they are the same, you can replace it with a standard ballrace called a 5303. However you might do better to use a sealed bearing instead of an open one, so that lubrication is assured. Then it would be called a 5303-2RS. Just look that up on the internet and you'll find several vendors available.
To remove the bearing, the usual method is to remove the sprocket, then use a gear puller to pull the bearing off the shaft.
