RE: Which 4-Stroke for Sig Senorita?
A Saito .56 on a Sig Senorita? Why? If you are a fan of Saito engines and you want to owe and install one of them in your plane, you'd better pick up a more suitable plane than a Sig Senorita.
A Saito would look like gorgeous on a warbird, for instance. Also a scale trainer, like the Chipmunk or the AT6 Texan, would be a great choice.
Anyway the ARF's market virtually offers everything, all you need is to take a look around.