For an LT-40, if you fly from grass, and if it's not kept putting-green smooth, a 2-stroke .46 is a good engine. Off of pavement or very smooth grass a .40 would be fine. O.S. engines are great, but fairly expensive, though the LA series is somewhat less costly than the others. I've had good experiences with Tower Hobbies and Thunder Tiger 2-stroke engines, though some people have reported problems. (I've also gotten good results with Super Tigre engines, but they are not necessarily as user friendly as some of the others).

I wouldn't recommend a four-stroke for your first glow engine. Nothing wrong with them, and they sound great, but they're a bigger investment, a little harder to set up, and more likely to hurt you badly if you mess up.