AeroWorks has some good ones that will fit the bill. One thing to remember is the YS91 has as a lot of power when run on 30%. I would look at something that was close to the weight of the plane the YS came out of and start there.

I'm thinking the AeroWorks 60-90 Yak or Extra fits the bill quite nicely

http://www.aero-works.net/store/

I had a YS91 in an older Goldberg Ultimate a few years ago and what a power combo that was. I then bought the YS110 and then it became ballistic. Don't remember the weight though