is there a reference on the structure of the space of metrics on a compact manifold that induce a given measure $\mu $?

i have a given manifold $M$, a given measure $\mu$ with an everywhere positive $C^{\infty}$ density with respect to Lebesgue. i want to construct a metric in this class satisfying some additional properties.

more precisely, for a given metric $g$ inducing $\mu $, i want to keep a finite number $n$ of eigenfunctions $\{ f_i \}$ of the laplacian, $$ \Delta _{g} f_i = \lambda _i f_i $$ Then, i generate another family of $n$ functions $\{ f'_i \}$ as linear combinations of the $\{ f_i \}$. What i want to do is to construct a metric $g'$ inducing the same measure such that $$ \Delta _{g'} f'_i = \lambda ' _i f'_i $$

I am looking for coordinate-free (if possible) calculations including this kind of objects.

thanks, nikos