On a two stroke, I worry more about trapped compression than full stroke compression.

Think about it. Let's say that your NSR has a difference of 1mm of exhaust port height between the two cylinders, but everything else is equal, then the full stroke compression will be the same, but the trapped compression (when the exhaust port closes) will be different.

You need to know:

Head volume
Bore diameter
Stroke (from exhaust port closed to TDC)

When you know these numbers you can use the calculator link