Some people have the fear that modifying their engine will make it too fast and too unreliable. But here's what I want you to know;
Opening the engine ports can make it faster but you can lower the speed back to normal with a larger rear sprocket and then you will have a bike that accelerates really good and climbs hills better. But maybe a larger sprocket isn't necessary if you put on an intake extension because that increases low rpm power while taking away from high rpm power.
And the only reliability issues are with two inexpensive parts: 1) the upper connecting rod bearing which you can easily replace at a cost of $6, 2) the primary gear woodruff key on the crankshaft The screw holding the gear on needs to be loctited and checked regularly. If you port for racing then you may want to make a steel woodruff key from a bicycle sprocket (make it 8x3.5x2.3mm) or buy a set of 25 alloy steel ones (40% stronger) from McMasters for $5.85. The part # is 98525A060.
When you lighten the piston/wrist-pin you will actually increase reliability because that eliminates engine vibration. And replacing the stock CDI with the Jaguar CDI greatly improves reliability because the stock CDI fires too early which causes excessive compression (stressing the crank and con rod bearings) and vibration (which loosens all engine nuts/bolts).
Concerning expansion chambers, the top speed can be set by the header length. You can lengthen the header for less top speed and more low rpm power. You see, you are in control. With these small engines it takes all the modifications I recommend to really get some decent power out of them. Making one little change will not turn it into a fire-breathing monster overnight. I wish it were that easy but it isn't. Doubling the power of these will still leave you with just  a few horsepower.

With increased power you may need to increase the tension on your clutch pads. That is done by tightening the flower nut, pulling in the clutch lever, and then tightening it one or two notches more. Then readjust the clutch cable free play.