I had a .46 FX on mine, it was still nose heavy even with the receiver battery clear in the back of the fuselage behind the back of the servo tray.
The Magnum XLS .46A is a great sport engine, but it is still a bit heavy for the Tower Trainer .40 MkII ARF. I like plain bearing engines myself, they're extremely realiable and they run great so long as you use fuel with plenty of castor oil rather than 100% synthetic lubrication.
If you want a ball bearing engine that will fly the Tower Trainer .40 MkII right without being too heavy, look at a Thunder Tiger Pro .36 or Evolution .36NT. These weight in around 13 ounces or so with muffler and will pull the Tower Trainer .40 MkII around with plenty of authority.
Edit: The Magnum XLS .36 2-stroke or Super Tigre G-34 would also be a good choice, I just had to double check to make sure Magnum offered one in a .36-size!
