In general, on either 4-strokes or 2-strokes, anything that can be done to make the "pump" (engine) have adjustable parameters will help optimize performance (and often emissions) for particular rpms.
My background is in 4-strokes, but there are a lot of analogies betweeen 4's and 2's as it all goes to the fundamentals of the engine being a pump.
Hot-Rodders in the 70's used Rhodes Lifters. These were bleed-down hydraulic lifters that provided variable lift, less at lower rpms and more at higher rpms. This was mostly used as a patch for people who made a poor choice in selecting a cam (they selected too big of one that made the engine run terrible at low speed and threw these in to try to compensate, been there myself).
Honda had the V-Tec (sp?) variable valvetrain in the 80's. That also provided variable flow at different rpms. Search the Internet for details on that.
A really cool, but slightly different concept, though worth mentioning, is Cadillac's 1980/81 8/6/4, which never really took off, but just resurfaced in a car last year. Concept is that 8 cylinders get you to highway spoeed, then once the inertia is overcome, it reduces to 6, then 4 cylinders at cruising speed to reduce the fuel usage and emmisions output.
The port timing that you machine your ports to in a two-stroke is analogous to cam selection in a four-stroke. Problem is, to change port timing, you have to machine the cylinder.
One way of adding variablility to the two-stroke "pump" is through these valves.
As an all-encompassing statement for your report, you might state something to the effect that traditional engines (2 and 4) have fixed parameters (lift/duration/port timing/runner volume). As such, they are optimized for operation (power and emmissions) only over small rpm bands. Anything that can be done to make these parameters variable can increase the optimum rpm band.
Also on the topic of emissions, horsepower has also been directly related to emissions. More horsepower came at the expense of worse emissions. If you recall (or in your case, look in the history books
) the dreadful 70's, automakers fought the increasing prices of gas and the economical Japanese cars by simply taking our big cars and reducing the horsepower output, thereby reducing emissions.
You might consider doing a short study of the Corvette engine specs over the period 1965 (or so) to present. The small block Chevy has been virtually the same, yet the changes in it's power and emissions output are remarkable. The ways that they achieved it are also remarkable.
Most people bawk at computers in engines. They are the single most important item that have led us to extract the maximum potential out of an engine by providing feedback from all points of the engine back to the central computer to allow it to make a decision to adjust parameters.
OK getting a little off-topic her, but this is fun. I wish I did a report like this in school.
Good luck.
