I had a OS91FX in mine but the muffler was a huge problem. It wouldn't fit in the cowl at all and just made the plane ugly as sin. I have a 1.20 OS in it now. If you must use a 2-stroke, let me say this. The tank must be placed in the right position. If not the thing won't run worth crap. You can get a muffler from slimline that will wrap around back but its half the cost again of your engine. The sound of a four stroke is far better and the actual performance more scale. They can be flown like a Extra 300 but for my personal taste, I like to see them soar around scale like. You can't go wrong with the reliability and thrust of a 4 stroke, they are awesome.