The OS 160 with a Pitts muffler is a very good combo of power and light weight for that plane.

I'm tempted to try an XYZ 50 on it from http://www.southeastrcengines.com if the Brillelli ever gives it up since I suppose it won't be easy to get worked on. There is a guy at our field (excruciatingly slowly) working on that combination so I'll get to see or fly it for myself.