Hi,

Sorry I haven't got any reply and my plane hasn't been in the air since then. But i think you are right, a 120 size four stroke would probably work better and the large size of the wings surely could handle this engine size. But you probably need a bit counter balance weight in the tail which is a pity. Do you still have the instruction manual? If so, it would be nice to know where is the Center of Gravity?
Actually OS 61 FX delivers 1.9 BHP at 16,000 RPM and weighs only 23,6 oz with muffler. The YS 110 (can't find any 120 size) weighs 28 oz but I can't find how much power it delivers. But a 120 size OS 4-stroke delivers 1.9 BHP, same as your 2-stroke OS and weighs 31 oz. I don't think the YS with a smaller size delivers much more than the larger OS so you would probably not gain much except a nicer sound. As your plane already has a 2-stroke, perhaps it would be better to increase to a 75 size 2-stroke like Supre Tigre delivering 2,18 HP and weighing 27 oz. Or actually, if you choose Supre Tigre you might as well use the G 90 which weighs the same (believe it has same size) but delivers a hefty 2,5 HP. Or even an OS 91 FX which delivers even a heftier 2,8 HP at a bit lower weight of 24,3 or almost same as your existing OS 61. With this you probably also would not need to change the cut outs in your cowling. Please let me know if you changed your engine and how it worked.