Hi!
What you do is this...you put a neopren rubber collar on the neck! That way the tubings will not touch the engine backplate.

...You must be joking! A .91 two stroke on a high winged trainer!!! And your a newbie! This plane is more suited for a .40 engine, at least at sea-level!!
Forget about throttle management when your a newbie! It doesn't work!
Choosing the correct size engine is what is important! People new to this hobby always think that putting in a too big engine can be compensated for by using throttling management...it isn't so (that easy). Been flying R/C and training newcomers for 32 years.