A diesel will run faster with bigger bore up to a point, just like a glow engine will. I think the "up to a point" may be smaller for diesel than glow, but don't know. How much power an engine puts out is directly related to rate of airflow through the engine. I'd fly the airplane with about 9 x 8 on glow, then try it as a diesel on the same prop, or maybe a 10 x 8. Don't know about pipes.