The best engine for the Tower Trainer .40 MkII ARF should weigh in around 12 to 13 ounces with muffler. Anything heavier than that will make it difficult to balance properly and the plane will be very nose heavy. Good choices for this airframe in no particular order include:

-Super Tigre G-34
-Magnum XLS .36
-Evolution .36 NT
-Thunder Tiger Pro .36
-Thunder Tiger GP-42
-O.S. Max .46 LA

The new O.S. Max .35 AX will be a good fit once it's available.

Engines that aren't good choices include most every .46 ball bearing 2-stroke on the market. The Tower Trainer .40 MkII ARF is a lightweight airframe that doesn't need the extra power to fly fast and it won't balance properly with a 16-to-19 ounce engine on the nose.

Good luck and good shopping!