I'm afraid I don't know the compression of the specific engine you're asking about but I think (on the basis of my 35cc engine) it would be somewhere in the range of 5:1 to 7:1 trapped (from exhaust port closing).

To avoid the problem of chrome peeling off during porting, you have to be careful about which way the carbide bit is spinning. It should always be in the direction of the teeth on the bit, and you should use a very high RPM with very light hand pressure on the die grinder. Or you can use a fine emery bit to achieve the same effect if your die grinder cannot be reversed to match the carbide bit.