My newly finished one has an OS 61FX on it but it will be spring before I can fly it; I can tell you there is no need for a Pitts type muffler.
The engine is canted to allow the stock muffler to nestle underneath, and they've built in a "tunnel" into the fwd fuselage area to accomodate the muffler. Now, that does require cutting away a lot of the bottom of the cowl; if you want to use a Pitts muffler there is room under the cowl...but they have already mounted the blind nuts for the engine mount with that offset. To use a Pitts muffler you would need to change that setup so that the engine sits at 90 degrees and the pitts muffler hangs directly underneath.

I fly JR so can't help you on a Futaba servo but there's no need for the top end, high dollar digitals on this model...I run JR Sport ST126's which have 126 oz-in of torque and run about $35 each. I'm sure Futaba offers something in between their competition servos and the cheapy standards...that would be all you need.