A .61 is too big for a LT .40. A .46 size engine would be better.
Since as a newbiew you are at risk for crashes, cheaper 2 strokes would be better than a 4 stroke.
Check out
www.spadtothebone.com and look up plans for the Debonair trainer which you can build for about 20 bucks in material and is much much more durable than balsa.