A .46-size ball-bearing engine like the O.S. Max .46 AX or Evolution .46 NT would fly an Alpha .60 ARF trainer just fine. Extra power wouldn't hurt anything, however, and it might make the plane easier to fly in windy conditions. Engines get much thirstier as you move up from a .46 or .52 to a .61 or .75. A smaller engine would allow for longer flight times before having to land to refuel.

A two-stroke in the .52 to .61 range or a four-stroke in the .70 to .82 range would be a nice compromise between power and fuel economy. You might check out Ed Moorman's thread in the glow engines forum on the excellent JBA .56 2-stroke engine available from www.kangkeusa.com for just $79.99:

http://www.rcuniverse.com/forum/m_6762745/tm.htm

Ed compares the JBA .56 to the new O.S. Max .55 AX quite favorably, even though the JBA engine barely costs more than half the price of the O.S. model. Power, economy, and $70 extra dollars to spend on servos and a receiver; how can you go wrong?