To calculate the Cr you need to know:

The Bore and The Stroke (to work out swept volume)

then for unswept volume you need:

The compressed volume of your gasket
The volume in your head (the bit you are changing)
The dish of the piston
The deckheight of the piston




CR = (Swept + Unswept) / (Unswept)





If you can retard the timing you can make it safe to use mate