I own one of these with a .75 2stroke, very fast plane. I take it out when I feel need for speed. I've built 3 of these with 4-strokes, although the YS is a lengthy engine. I've always cheated on the thrust line and stretched it a bit, about 10mm ahead of what that thing WorldModels calls a manual is calling for. The cowling on these planes is very forgiving in engine size and placement too.
Also something else comes to mind, I'm not sure if he was in on it, but there's a user here that comes from an area north of me. The club I heard was(is) racing these things with YS. 120's I heard.
MGlavin is his screen name. Lets see if he posts here. I'm almost sure he could tell you for sure about the YS installs.

Hey Michael, whatchya got man??