Rule of thumb is you want at least 100W per pound for sport flying. For Aerobatic or 3D flying you'd want 200W+ per pound. Look at the RTF weight spec'd for that plane (with nitro setup) and use that as a basis for your power calcuation. Then find yourself an outrunner with your desired continous power rating, and look at the motor data for prop sizes, batt voltage, and current draw. Low Kv motors will spin larger props than higher Kv motors with the same power rating, so you want to get a Kv rating that will let you spin some prop sizes that will work well with your LT40.

Once you know the power you need, the motor and prop combo that works, then you'll know the battery pack & ESC voltage/current requirements, and you can buy the appropriate parts.