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A model for S-wave $\eta\pi$ scattering is proposed which could be realisticin an energy range from threshold up to above one GeV, where inelasticity isdominated by the $K\bar{K}$ channel. The $T$-matrix, satisfying two-channelunitarity, is given in a form which matches the chiral expansion results atorder $p^4$ exactly for the $\eta\pi\to\eta\pi$, $\eta\pi\to K\bar{K}$amplitudes and approximately for $K\bar{K}\to K\bar{K}$. It contains sixphenomenological parameters. Asymptotic conditions are imposed which ensure aminimal solution of the Muskhelishvili-Omn\`es problem, thus allowing tocompute the $\eta\pi$ and $K\bar{K}$ form factor matrix elements of the $I=1$scalar current from the $T$-matrix. The phenomenological parameters aredetermined such as to reproduce the experimental properties of the $a_0(980)$,$a_0(1450)$ resonances, as well as the chiral results of the $\eta\pi$ and$K\bar{K}$ scalar radii which are predicted to be remarkably small at $O(p^4)$.This $T$-matrix model could be used for a unified treatment of the $\eta\pi$final-state interaction problem in processes such as $\eta'\to \eta \pi\pi$,$\phi\to\eta\pi\gamma$, or the $\eta\pi$ initial-state interaction in$\eta\to3\pi$.

Form factors of the isovector scalar current and the $\eta\pi$ scattering phase shifts