Hey Jim,
The rear exhaust would require some surgery to the fuselage , probably more
trouble than it would be worth and since I'm not an OS fan that one would be
eliminated if it was me. Eddie had a YS 91 in his originally and I understand it
flew really well. The 1.20 Webra would probably be good also as there is plenty
of room in the engine compartment and I like Webra engines and I also like plenty
of power which the Webra has . You're going to enjoy this airplane.

Good luck
tommy s