There's 2 ways to cool an engine with the fuel mix... more oil or less nitro.

The easier way to cool it (assuming it has adequate airflow) is to open the needle valve a bit. You should be tuning for maximum RPM then opening the main needle appx 1/8 to 1/4 turn.

Not a normal problem for a trainer.... If the engine is heating due to being cowled and not having adequate airflow. Changing fuel or tuning the engine won't help for more than 10 extra seconds. You need to ensure you have at least 2.5 times the exit air opening size as entry opening. (The preferred ratio is usually 3 times as much exit as entry.) You also need to be sure the airflow route will have the air going over the engine head and cylinder. The head and cylinder need 9 times the cooling airflow that the crankcase needs. (can't have the crankcase get 0 cooling air...)