To be honest, your best bet may be to migrate of ASPX+Portal. I would check the "ASPX" pages that the templates run off of, if there's little to no custom coding in them, you often can just copy the structure (with a couple minor modifications) into a Page Template's Layout (on portal mode).
The header/footer you'll have to take a look. I've migrated a couple sites off ASPX before and usually was pretty painless, and greatly improved the site's manageability since we no longer needed to go into the code to make adjustments.
Otherwise in terms of a Mobile accessible site, many sites are now responsive which allows them to be usable on both Mobile and Desktop, without needing to manage 2 different sites. This usually requires a pretty extensive site overhaul though...