J,
Ryobi's come in single and two ring versions. There are two shaft versions, long and short. Many have vertical slots cast into the cylinder wall above the transfer ports. These make the weedie easier to start, but reduce topend power. Some older models have tinnner con rods, which don't hold up to high RPM as well. Some have smaller diameter countwer weighs and crankcase inside diameter. There is some variation in the backplate and reed designs. This effects case volume. The port timing is about the same on all models. Exhust duration is 150 degrees and transfer duration is about 110 degrees. Of course the intake timing is variable, because it is a reed valve. Thats about it.

Dave