Thibor,

Out of the three that you suggested I would probably go with the 61fx. simply because it will give you more of a pullout than the .55 (and certainly more than what i currently have). Personally I think a .75 would be total overkill and just add weight.

If I had the money, I would certainly go with the .82 four stroke out of O.S. I think that's the PERFECT combo for this plane, and it's what I will certainly get once I get the chance.

Have fun with the plane and you will certainly not regret purchasing it