That's a tough question. Kha is a major ingredient, responsible for most of that wonderful delicate aroma and flavor. It's not going to come out the same if you use ginger. I've always thought fresh ginger to be more forceful than kha, so I'd try half the amount if using ginger. Check it at end of the first simmer (before adding the chicken and other ingredients), and add more ginger if it seems necessary.
P.S.: You can get fresh kha, and some other Thai produce items, here
. Kha comes in two-pound packages. You can wash and peel it, cut it into reasonable chunks, then freeze it. Instead of freezing, I store it in a jar steeped in cheap vodka. Either way it'll keep for months. After it's been in there awhile, you can also use the vodka to add a bit of galangal flavor to other dishes. I use this method to preserve fresh ginger, too.