I've flown the Tower Trainer .40 MkII ARF. It's a wonderful airframe, but a typical .46 ball-bearing 2-stroke engine will make it very nose-heavy, even with the flight pack battery glued clear back into the tail section behind the servo tray.

The .40 LA has been discontinued by O.S., but the .46 LA is almost as lightweight. The O.S. Max .46 LA or Thunder Tiger GP-42 2-strokes are both excellent choices and relatively inexpensive. They will both also provide plenty of power while still allowing you to balance the plane on the recommended CG without having to add weight to the tail.

If you decide you'd rather have a ball-bearing engine or a 4-stroke, consider a .36 2-stroke ball-bearing engine from Evolution or Thunder Tiger, a Super Tiger G-34, or any of the .52~.56 4-stroke engines available from Saito, O.S., Magnum, or Thunder Tiger. The Tower Hobbies .40 MkII ARF is lightweight and has plenty of lift, any of these engines would allow a full range of aerobatics while meeting the weight requirement for ideal balancing.

Good luck, and good shopping!
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Oops, I'm an idiot. I swear the part about you already owning a .40 FP wasn't there when I first read the thread!

The .40 FP should be a terrific match. You'll want to try a 10x6 and an 11x5 prop to see which you like the best. The 10x6 will give you a little more speed, the 11x5 will be slightly slower in the air, but will give you quicker take-offs and slower landing approaches.

The .40 FP should provide plenty of power for inside loops, barrel rolls, wing-over turns, and all of the other standard aerobatics that you will want to practice.