Praise be to Allah.
We put this question to Shaykh ‘Abd al-Rahmaan ibn Jibreen (may Allaah preserve him) and he replied as follows:
We think that if it will not cost too much to repair it, then a person will be rewarded for restoring it, even if he is not one of the people who are in charge of the waqf, such as if he sees a mosque that is about to collapse and he repairs it, even if that costs the same as building another mosque or half of that, because in this case it is as if he is sharing in the maintenance of the mosque. The same applies to charitable schools or charitable waqfs (I mean a building that is for us by a charitable organization or the like).
And Allaah knows best.