You can force the Moon Stones and the Cerulean Cave Fossils to respawn by using the date change trick shown here

Note that the Moon Stones are not guaranteed to respawn with this method, but it shouldn't take many tries. There is also one more Moon Stone in Copycat's house in Saffron City. Generally it's more consistent to do the respawn trick than catching Clefable. (This last part might be slightly outdated considering the newfound knowledge on the catch formula)

There is a separate Category Extension for that! https://www.speedrun.com/pkmnletsgoext#Solo_Diploma

wartab, big fan