I understand.

4-strokes are far more fuel efficient than 2-strokes, greatly due a design that harvests power before releasing exhaust (which also makes 4-strokes cleaner burning, more environmentally friendly). There's really not much room for improvement in the 4-stroke exhaust system.

So, your original comment is absolutely valid - about the only thing an exhaust change can do for a 4-stroke is change the sound.

2-strokes produce much more power per CC, but much of the potential is lost in the inefficient exhaust design. So, improving the exhaust system of a 2-stroke can improve performance (but not do much, if anything, to improve fuel economy or make the exhaust more environmentally friendly).

A scavenger exhaust system (a generic term, not a brand) sort of sucks some exhaust from an expansion chamber back toward the cylinder to increase compression. (Warning: Try that on a scooter with inferior engine metals and it will burnout/shorten the lifespan of the engine.)

It is not a matter of just buying a scavenger and bolting it on. The scavenger must be matched to the CCs and the carb (mainly the jetting). IMO, using a scavenger on the little automatic carbs that come with most small scooters is a waste of money, like putting a bowtie on a donkey in hopes it will become a race horse, or in my case, a plow mule.

Most of my riding is on a 150cc 2-stroke LML manual shift (same basic make as the Genuine Stella sold in USA). It might benefit from a scavenger but doesn't really need it so I have not bothered.

But I use a 49cc 2-stroke TGB (well designed, well manufactured, light weight, quality scoot) as semi-dirtbike in the East African bush in volunteer work with the national wildlife service in expansive wildlife reserves. No paved roads and mostly narrow game trails.

First I ripped out the little Mikuni automatic choke carb and installed a Dellorto racing carb. Much better performance, but it is not called a racing carb because it is so fast; think pit-stop ease and quickness of cleaning and adjustment. Also upgraded to a more durable, more reliable (moisture and dust resistant) performance CDI and ignition coil.

Then* it was time for the scavenger, which must be carefully matched to the CCs and the carb jetting. Good scavenger manufacturers have charts for that and will provide assistance.

The guys that assisted me even recommended a the best jetting and variator adjustments. That comes down to a choice between higher top end at a significant decrease in acceleration, or max acceleration with a bit loss of top end. Because of where and how the scoot is used - seldom top end - my default is for acceleration. But I also carry the top end weights, which can be popped in quickly if I will be riding on a paved road.

I always pay a higher price for a cleaner burning "smokeless" type 2 oil because I feel so damn guilty about air pollution.