ORIGINAL: HighSierra
I've got a YS 91AC, but I'm thinking of selling that along with a OS 61 FX to finance either a YS 110, saito 180, or a OS 1.60..
A YS 110 would be plenty and fast for this plane. Plus, it's the same physical size as your YS 91AC, so it would fit nicely inside the cowl. If you are looking for speed and awesome sounds, then the 110 is your ticket. The Saito 180 would also work, but it has much higher fuel consumption and doesn't spin as high of an rpm as the YS 110.