ORIGINAL: Missileman
I wouldn't go higher than the .61 because it is an ARF and because it is a CMP ARF. The airframe may not be able to handle much more than that.
I disagree as I have an identically built CMP Extra ( it has a .75 engine in it too ).
The Katana 50 is a fully sheeted plane. It needs a slightly larger engine for best performance.
It will have no problems handling a .75 engine, and if anything I would recommend something like this.
However it does pay to be careful and re-enforce the tail.
Adding CF struts on the tail is cheap and easy to do.
The CA hinges could be pinned to add strength too, and the hinge area should be thoroughly soaked with CA for safety... using any of the standard techniques for doing so.
The tail gear should be changed out, and it would benefit from slightly larger tires.