I have a saito .72 four-stroke in my SE and it was a tight fit. I used my dremel with a drum sander to make room in the cheeks for the carb and high speed needle. You may need to make a modification similar to this for the .61 you propose.

On the other hand, if you do go with a four-stroke, you will be absolutely happy, as I am with mine. Unlimited vert with a .72 and a 12x8 APC prop. One thing to be aware of is that with the longer diameter props that four-strokes are capable of swinging, you may need to get taller landing gear and/or larger diameter tires.

I used the recommended 2 and 3/4" tires, but used the taller landing gear from TNT.