I'd have to agree with Mike... The 91 FS isn't a good fit for the Mojo 40 ARF.... I just thought since you were asking about Mojo ARFs, I'd let the cat outta the bag...
On a Mojo .40, you're better off with a .50 class 2 stroke or .70 - .82 class 4 stroke... The Katana .70 is an awesome plane and the 91 would be right at home on it....