Perhaps blasphemy, but GM
Perhaps blasphemy, but GM makes a fine engine 4-cylinder that would take the Miata under 5.0. It's the LTG 2.0 cylinder, good for at least 300 hp.
I should know, I drive it's kissing cousin the LNF in my Sky Red Line, and that gets me under the 5.0 mark with zero mechanical modification.
If Fiat can put its engine in an MX-5, why not GM too?