I have this airplane and power it with an O.S. FX 61 with an APC 12/6and the performance is excellent!! I think you will not get the full potential from this aircraft with a 52 size 4 stroke engine, If you have to use a four stroker in it , I would most definetly put the O.S.91 in it. However you might have a problem with prop clearance. I use 14/6's on my O.S. 91 4 strokes. I believe it says you can use up to an 80 size 4 stroker in that airframe so the 91 isnt to far a stretch. I would say go withthe O.S. 91.All my Nitro engines are O.S.I will never stray from the best. You will love that plane, it flies GREAT!!!! Rotten out!!