so I did a little comparison since I got a second iso mount for my OS 46AX for my Gizmo kit build which works great on that plane as well as being inexpensive it does reduce vibration through the airframe.
the OS 46fxi and the AX engines are nearly identical other than the P-box is a little larger toward the outside of the exhaust flange the dimentions are the same for both engines.

I am planning to switch the engines since I am planning on mounting the AX in a Great Planes mustang kit I have,really there great engines the AX just has a little more umph to it.
the comparison pic is a little off angle, I did messure both engines and there nearly identical.

between the nose gear and the engine mount issue you might have a solid reason for a exchange of your fuse or the entire plane,the Nextar is a nice trainer,and has a good looking airframe,sometimes these ARF's don't get done corectly Tower is pretty good with problems