RE: Kat 40 Dude!!
CG I'd say sure but, its been my experience a 4 stroke is just better for 3D type planes. I have a saito 82 on the way for mine. I want a 15x4 for tons of prop wash on my kat GS 40. I use a oS 46AX with a 12x4 now, it works fine, but wont pull from a hover with authority. Since you already have the strong 50, I say try it. That engine should swing a 13x4 fine.
If I remember right you had or have a twist, The KAT is so much nicer IMHO.