the OS 46AX and 55AX will fit in the supplied engine mount provided in the ARF kit, the iso smooth mount works well,I use one in a sport plane I have.
the OS 46 works fine with the NEXTAR airframe, if you go with a larger engine you going to have to shift the battery and reciever rearward to get the CG to balance since the plane was designed around using the OS 46.