ORIGINAL: maynardrupp
If you are considering the OS46AX, for a few bucks more, the OS55AX is a more modern engine and fits in the same mount. I have had both in SIG Somethin' Extras and the 55AX is clearly a better engine with a lot more available power. It starts, runs, and idles better than the 46.
If you want this plane to have unlimited vertical, the OS 55AX is the engine I would recommend for power Vs $$$. However, I have flown a 6 pound Trainer 40 for 8 years on a TT 46 PRO SE (as it was called back in the early 90s). Plenty of power and I could have flownthe Sportsman sequence.
I currently have an OS 55AX in a ZEN 50 w/ unlimited vertical on a new (not broke in) engine and it weighs 6 pounds.
Again, I believe the Magnum XLS-46, OS 46AX, Thunder Tiger 46 Pro, and Super Tiger GS-45 ABC will fly the Kaos 40 well for trying out pattern. Most economical buy is the Magnum XLS-46 as Hobby People has it on sale for $49.99.
I will probably be getting one for my wife...
LLD