You can likely replace those tuners with "Ping" brand tuners but don't give up on the originals yet.
Here is what I would do, and have done to make those tuners work great:
Take them off the uke, clean them with a rub down of Brasso if you want the metal to look new again. Then, really clean the screws, especially the threads (I twist them in steel wool) and when clean, put a drop of lubricating oil on the threads. Next, twist the screw into and out of the threaded shaft by hand several times while not on the uke. Wipe off any oil on the outside of the shaft. Then, one last step. Put a small galvanized or brass washer between the back of the headstock and the plastic tuner button when you reinstall. This allows the plastic to slide over the metal washer more smoothly as compared to being against the wood headstock, and it allows a bit more play in the threads when screwing in the old screw, so the screw doesn't sit as far down in the shaft.
This should make them work better than new!
By the way, great score for $35!