As noted, knowing what aircraft, engine, prop you are working with now is probably the first step.

From there, the optimzation is the best balance between airframe/engine/exhaust system/prop

For most models, the aircraft itself is the limiting speed factor. Frontal area is the key contributor to drag. Cooling openings, parts out in the breeze, stuff like that add to drag very quickly.

Engine wise ..... internal mods on current engines typically do not yeild very much. Adding a tuned pipe (properly tuned for the aircraft and prop used) or adding a tuned muffler like the Turbo-Jett will give you 15-30% more HP on average depending on the application.

What is important to understand, is that, with a given aircraft, it takes a lot of HP increase to result in a small speed increase.

But its good to have people out here with the need for speed, and yes there are things you can do to add a bit of velocity to just about anything!

Bob