If you watch for a while on Ebay, you can get a full set for not much more. If you are going to the expense/time of displaying them, you may as well get something that looks good and complete.
If anyone wants old balls like that, I have a bunch of extras including full sets. Shipping is going to be about $15. The cost of the balls depends on condition, but I think I could put a complete set together for $70 plus shipping.
Even if old clay balls are stained, you can make them look pretty good by sanding, polishing and waxing them. The color goes deep.
The balls in the pictures don't have many cracks. I think cracks give them character.