Worn out rings will not change the compression ratio. compression ratio is calculated from the difference between volume of the cylinder when piston is at bottom dead center compared to the volume of the combustion chamber. This stays constant even as the engine wears. the only thing that changes compression ratio is by changing boar, stroke or cylinder head volume. Check your compression pressure. Put a compression gauge in the plug hole and crank the motor approximatly six revolutions. that pressure reading is a good number to use. There is problably a spec in the repair manual for that motor. If you mill the head you will raise this number. You dont want super high compression pressure on a two stroke. The compression ratio of 6.4 to 1 is ok for a two stroke. If you really want more power you should get some new rings and hone the cylinder. Make sure everything is in perfect shape then clean your carb and make sure your ignition system is in perfect working order. After all that is in perfect shape you can start by changing pipes or porting. Just do your homework or you will end up with a boat ancor. Get gordon jennings two stroke tuners guide and start reading.