Of those three options I'd prefer the YS 1.10 as it fits like a glow, it's the engine shown on the plans. But if you look at price you'd get the 91 FX setup for half the price of the YS and the O.S does have more than enough power for this plane. Do NOT think about stuffing a 1.40 in there, there's not enough room and the plane is just not built strong enough to handle it. I though about the O.S 1.20 surpass III but a quick calculation showed there's not room inside the cowl for a 1.20 engine [8D]