The horsepower ratings are a guideline but they can be a little misleading. You'll notice that the advertised HP is at different RPM on different engines, usually at an RPM that it will never realistically turn. The true test is how much torque it makes and how many RPM's it will turn the prop being used. I have a Super Tigre GS-45 that has a lower advertised horespower rating than an OS .46-FX I used to own. The OS ran excellent but the Super Tigre turns the same prop at higher RPM and the plane performs better. Truth is, just about any good .40-.50 sized engine will work great on your LT-40. Just make sure it is broken in well and is tuned right. personally, I'm partial to the Super Tigres. I know some people think they are cheap but mine have always ran great. I've been impressed with the TT Pro .46 too. I have a tower .46 that I wasn't very happy with at all. Then a guy at our club suggested some 15% fuel, it suddenly became an awesome running engine.