Thanks Dave. That was very helpful.
...Many have vertical slots cast into the cylinder wall above the transfer ports. These make the weedie easier to start, but reduce topend power.
I'm worried about this... It can mean a important power decrease. In fact, it's the first time I hear of such thing in a small engine (the kind of "decompressor" I knew was a valve on top of the cylinder in some small bikes, which you pulled with a steel string). Is there a way to find out that without opening the engine?