If you are talking serious 3D, you do not want to put a .60 size 2 stroke or .90 size 4 stroker in there. To much weight penalty, and therefore to high of a wing loading.
I am not saying that the plane couldn't use a 61FX, but you can get power also through a different way.
I had a OS 50SX with regular muffler (baffle removed) on mine, and it was lacking that extra bit of escape power on certain maneuvers or hovering. After trying all kind of different props, I still wasn't happy and decided that I needed a different exhaust system. The problem you have with this plane is that not many tuned exhausts will fit it due to the wing (Ultrathrust muffler won't fit for example). I did find however the perfect pipe (I believe) in the Mini Rossi tuned pipe available from Morris. It now has so much power, that it really shoots out of a hover, and you have to slow it down. Really ! Unbelievable power increase. The neat thing is that you can tilt the muffler up and down, so wing clearance is no problem. Midrange is also not a problem with this setup, and the engine doesn't jump on the pipe to soon.
My perfect combo for the Flip is: OS50SX + APC 12x4 prop + Rossi tuned pipe !
Just my $0.02