Does a mosque built on land bequeathed by a woman for that purpose, which then became too small and was donated additional space, become the property of the heirs or of public funds?

The old mosque has become an endowment (waqf), so it is not permissible to return it to the heirs of the endower, because it has exited private ownership. It is not permissible to dispose of it in any way other than that for which it was endowed, as long as its benefits are being realized. However, if its utilization ceases, it is permissible to sell it and use its price to build another mosque or to expand an existing one.