I have a 40FP on mine and love it. Can't go wrong. I have mine pizzabox at 24"x24" and a hard wood motor mount, and mines weighs in just under 3lbs, and my 40FP flys it great.

Look at the weight of the 40LA, is weighs approx 12oz with muffler. then look at what a 25FX weights, They weigh about the same. And since you already have the 40LA you don't have to buy another motor.