Many thanks for that very informative response techinspector1 ! your input is very much appreciated and is very important !

I am an automotive engineer so I know better than to guess at any of this and end up with money spent and something undrivable.
That is why I am taking the time to measure and calculate everything and get your expert advice before spending money.

I want to build this engine right.

All measurements were taken using the correct instruments.

Cylinder volume was obtained by measuring the bore with a bore micrometer and the stroke with a depth gauge.

Chamber volume was measured with a medical syringe.

Gasket volume same as above

Piston dome was measure by inserting the piston exactly 1" down the bore, sealing the crown to the cylinder and then measuring with the syringe and then calculating.

ring land volume was calculated from measurements.

These are the figures

Cylinder 958cc (109.38x102mm)
Chamber 115cc
Dome 41.6cc
Gasket 13cc
Ring land 1.1cc

If I take off 20cc worth of dome, I get 9.9:1 compression which is acceptable for 91 octane ?
I can use a thicker head gasket when I assemble so that will help.

I can start off by cutting the piston 90 degrees to center-line but I would be cutting from the sharp edge of the dome, thus removing very little material per depth of cut.
I would need to cut parallel to the flat angled surfaces to take off 20cc worth of material.

What do you say techinspector1 ? should I mill them first 90 degrees as far as possible and then along the angled plane to arrive at the required dome volume ?