I've had 5 UCD, most with invert mounted angines, one was a 4-stroker. I wouldnt hesitate to put OS 1.20AX(or any 1.00+size 2-stroke) in the .60 Do, even if it came out 8.5lbs- 9lbs, it would be great pull out power. Most inverted engines can siphon fuel into the carb, but I found that if you make sure the carb is fully closed while fueling, it is minimal. Then when its time to start , you dont even need to "prime", especially the 4-strokers. If I forgot & primed the 4-strokes, I often got "fuel lock" from a flooded engine. A Cline fuel system will also eliminate any siphoning problems. The fuse shape is a natural for an inverted mount, it fully conceals the engine in the cowl. I had one side mount in the Do, a Moki 1.35, I had to cut a lot of cowl, but it worked well, plenty of power to hover. I'm kind of surprised that 4-strokes are still the first choice for many, after having several, I went back to 2-strokes, there was no special advantage to a 4stroke, in fact, a good running 2 stroker is a better for 3-D IMO, then you save $$$$ on fuels & engine. Just IMHO