How can the intended man be found after 20 years to deliver what is rightfully his to him, and is it permissible to donate the sum or use it to build a mosque if that is not possible?

If a person has a right and despairs of attaining it, or for his heirs to attain it, then it is permissible to give that right as charity on his behalf. This includes contributing to the construction of a mosque. Any money whose owner is unknown, or whose owners are despaired of being known, should be given as charity and spent on the welfare of the Muslims.