Thunder Tiger GP42 , best kept secret in model aviation. Reliable , powerful but not too powerful and cheap. If you overpower your trainer it will most likely cause you problems. If you nose dive your brand new $$$ engine into the gravel then you are going to cry a lot harder than if you did it to a cheaper engine. When you move up to a faster plane get a bigger better engine and leave the easy to tune inexpensive one on your trainer for the future.