As far as power, you have many options. Personally, I prefer fuel or gas. The LT-40 will fly just fine on any .46 size 2 stroke glow engine. If low price is important, then the Thunder Tiger .46 Pro may be a good choice. Generally I have been partial to OS brand engines for most of 30 years.
They have a good .46 size engine. Fit, finish and overall design is second to none. But a bit more expensive. SuperTigre offers decent engines at a decent price. They have a .45 that is inexpensive. These are all 2 stroke engines, that are easier to maintain than 4 strokes, and are less expensive. Four strokes are nice, but heavier, more expensive, and require periodic valve adjustment. So, for 1st timer, I recommend 2 stroke glow. I have experience with all the above engines and manufacturers as well, and most 2 strokes now available will work fine for you as long as....

You follow the operators manual closely. That means:
Have a solid mount.
Use fresh fuel of proper oil and nitro content. Personally, I have great luck with Morgan Fuels Cool Power 15%. If you cant trust yourself to not run the engine lean, then please use Morgan Fuels Omega. It runs a little dirtier, but protects perhaps better.
Use balanced props in the recommended sizes. Most .46's will run fine with 10 X 6 up to 12 X 7(in the case of the OS) props. The 10 X 6 or 11 X 7 would probably be best to start with. Both wood and composite props will work. For a trainer this size I would use an APC brand prop.
Be patient and follow the manuals break in procedure.
DO NOT EVER run the engine lean. If you need more power, get a bigger engine.
DO NOT start the engine with your finger. Use a starter or a chicken stick.
(In the event you go with the OS .46AX, do yourself a favor and get a FOX RC LONG glow plug.)

There are many options with power. Part of the beauty of this great hobby. So many options. It can be hard to pick planes and engines. You have picked a solid first plane. To make the engine choice easy.....just get the OS .46AX! Of course, some will say, "go with a .61 engine now so you dont have to buy another one for your next plane". Well yes, but I would keep that trainer intact no matter what the next plane is. Its good to have at least two planes in the hangar.......

Congrats on your decision to build and progress in this hobby