To tune an engine is not always needed to change the timings .

There is always a lot of work to do in the crankcase . You have to asure that the flow is going fast without stucking .

To change the timing you will need to know the actual timings of your engine .Compare it with tuned engines on the market . Most of them are very closed to the limit of timing .

Pay attention on the closingtime of the crank . This can be bad for fuel consumption .

Polishing is not so good .Better rawing up with a fine sandpaper . This will encrease flowingspeed .
What I go to polish is only the intake of the crank. You can work the valve without changing timings . You can encrease the intakevalve extracting material and sharpening the edges. Than harmonize it with the crancase .

Lightenen the moving parts like piston and conrod will give you better accellerations and rpm. But there is a limit on all .
Do only lidle steps . One change after the other .

The pumping effect of your crancase is a very good tuning solution .
Try to reduce the volume . When you be able to do this you will begin to smile


And how reloaf said ......there is a very good book you can download as PDF .

You have to learn first how an engine works , than you understand what you can do .