Which version do you have ie. flat tailplane or fully sectioned?
My 14 year old son flies his on an OS91FX, with that amount of extra power it has to be carefully manged but it makes a cracking combination. If you have the sectioned tailplane and use a 40 sized engine you will need to add lead to get the balance.
Check out the video
http://media.putfile.com/Katana-40-July-05