I agree with you on all this, but why not the 91 FX instead of the 75 AX? The AX weighs 750 grams and the 91 fx weighs 770 grams. At 6.5 pounds the .75 will be marginal for 3-D and the fx 91 will be better. I fly my Katana 70 with a 91 FX and wood 15-6 prop now and it is just great. Idle and transition is great and the pullout from hover is ballistic (plane is 6 pounds). I use the Tower 75 muffler and that brings the total weight of the engine down to 700 grams (lighter than the stock .75) I just swapped out the YS 110 for the 91 FX and I like it as much as the YS in this plane.
I don`t feel that a fourstroke was any better than the two stroke

If the OS feels to expensive then the Magnum .91 is just as good (I have had that one too) and because the Tower 75 was weaker and about the same weight as my .91`s I ended up selling the Tower 75

I say go for a .91 twostroke if you don`t choose a bigger fourstroke