I have not had a problem running the engine in mine, but I did have problems with cowl assembly.
My original choice of engine was an MVVS .49, then a Super Tigre G.51, but I soon discovered that I was going to have to cut the entire front out of the cowl to get either of these to fit, as the carby's sit at a different angle to OS type engines. Also in both cases the cowl front would have fouled the throttle arm.
A switch to an old series 2 Magnum .46XL, swinging a Bolly 10.5 x 7, solved the problem.
The angle of the carby in relation to the crankshaft and head made the Magnum a neat fit. It is obvious that this model is designed around an OS type engine profile and dimensions.
But the trade off for me is performance. And as I do not wish to go and shell out any more cash for an OS AX or TT Pro, (I've got enough engines in the engine drawer as it is.) I'll just have to live with it. Fitting a GMS (Tower) type semi tuned muffler has helped a little.
As for how it flies? A marvellous model! Smooth, precise and very aerobatic, with no vices. Good glide.
Another at my local club has an Enya .54 4 stroke up front and it does everything mine does.
Jugman, what Nitro content are you using? One little trick with inverted engines is to up the nitro content a couple of %.