I have finely finished my accipiter and have flown it a couple of times. Let me first answer your question Bert, No I don’t think it will have enough power with a standard muffler. Let look at the pros and cons:
The plane was designed for a heavy engine + 900 grams. The OS91FX fits nicely with its standard muffler but you will need to ad about 120 grams of lead to the nose to get the CG right. I am running an OS91FX with a tune pipe, 14X8 APC, 15% Coolpower and getting 10400 RPM. I have used the 91FX without a tune pipe and got about 11000 with a 13X8 and a 14X6, I think the 14X6 would be your best bet.

The plane has no right thrust build in, which is a problem because it needs it. If I were to build the plane again I would not put the two servos in the tail, but would rather use a bell crank system and one servo in the middle, this would solve the CG problem when using a two-stroke.

Now for the good news: It can be made into an excellent sportsman pattern plane. With my setup (with 120 grams in the nose) it still weighs less than 3.8 Kg. It has more than enough vertical performance and I fly at 1350 meters. I did 5 flights yesterday and I don’t think I have ever flown a plane that is setup for pattern, what was this exciting. Because it's a big plane and does not have a very aerodynamic nose, one would think that it would be slow, but it’s fast. When horizontal I flew at under half throttle, it is only when vertical that I opened the throttle.