RE: a 13 pound plane with an os 91fx engine ?
The .90 is very light, but that means nothing, you will have to load up with lead to get the CG where it belongs.
Get any small 26 sized gas powerplant. Cheap to fuel and maintain. They can he had for far less than a Zenoah! I have a Z26 and a Brilelli 26, love both. The Brilelli is very, very light. You can get a chinese one for around $200...
Gerry