you can fill those holes in crankshaft with cork... it have some effect, and if something happens, cork is soft and it cannot do any dammage
![Very Happy :D](./images/smilies/biggrin.gif)
it wont lift your compression, but performances will be better.
the only thing you can do for better compression is lowering cilinderhead
ctx exhaust has limiter. inside is a longer tube, wich disable resonant effect... and it have metal grid allover the exhaust