Fortunately, largely as the result of Laszlo Toth's 1972 vandalism of Michelangelo's Pietà, restorers have come up with some pretty innovative means of repairing broken marble by employing mixtures of epoxy resins and marble dust which can fairly convincingly replicate the surface translucense of the original marble. It's a painstaking and expensive process, but fortunately it can be done if they can recover the missing pieces.
This aren’t valuable or rare statues calling for expensive restorations but we would like to have them complete, just the same.