If you don't own an SK engine, you shouldn't give advice. they are a totally different design from the norm.

-They have smaller transfer ports and less advanced timing. This was specifically done, according to what the engineer at Kangke told me, to make a high torque, low rpm engine. The ports are not big enough to pass fuel for high rpm. The engine will lean out regardless of a rich needle setting.

-They are ringed engines, but have a tapered bore like an ABC engine. As such, they should never be run in a 4-cycle or rich except for short periods during break-in. If you run rich, the sleeve won't open up and the ring ends will be pinched together, causing excess wear.

- They have a very specific break-in procedure. It works great for the 3 SKs I have (2-90, 1-50). A 15-6 prop runs excellent on the 90. (It's really a .91 disp, but they call it a 90.)

-They are easy, 1-flip starting engines, have good torque. They pull an Ultra Sport 60 around great. I've had 2 SKs on 2 US60s.

As for bigger displacement engines being heavier, let's not jump to any "obvious" conclusions. Virtually all .91s and .75s are now made in a .61 crankcase. The OS .61FX & .91FX weigh the same. The Tower .61 & .75 weigh the same. The SK .70, .80 & .90 weight the same. The ST .61, .75 & .90 weigh the same. The GMS .61 & .76 weigh the same. The new OS .75AX is going to be right at the same weight.

As for 4-strokes, my Saito .91s weigh less than about any 2-stroke .61. I would imagine, but I haven't checked, that the Saito 1.00 4-stroke is pretty light, too. The 1.15, I know nothing about.

For a Big Stick 60, I can tell you that a buddy has one and has flown it with a ST .90 and a GMS .76. Both are great power for the Stick. A .61 2-stroke in my first US60 made it a nice little trainer. A .75 or .91 does it much better. You can always throttle back.

I have owned most of the engines I mentioned except for the OS .75 and the GMSs. I do have OS .55AX & OS 1.20AX, so I have the .75 bracketed. For a fairly new guy, the OS will be the easiest to start, tune and fly. Little or no break-in is needed. If you have the money, they are worth it. For the guy who wants to try something different, the SK carbs are excellent. I have used them on Tower and Evolution engines and they run better than the carb that came on the engine. Their mid range is nice and lean. I have bench tested and run half throttle for a timed 60 seconds, then oppped the throtle open and had the engine jump to full power, no rich blubber, no staggering, no quitting.