If you are going to buy a single socket to do the job, sounds like 21mm is the one to get. Any reputable tool seller will trade you if it isn't the right one and the trade in looks good enough to sell to someone else. You might
find an electric impact to do the job. It would be cheaper to find a buddy with air tools like I did. Sometimes friends with air tools have full sets of metric impact sockets also.
I just went out to check mine. After I chipped the ice off the door and around the hood and finally got the hood up with its extra ice weight, the closest I got is a 15/16" socket. I know I went out and bought a single socket specifically to do that job, but I couldn't find it. I did find several old carburetors I forgot I had.
And my biggest socket is a 30mm deep impact that I bought to do the rear axle nuts on my bug.
Another pointer about doing the harmonic balancer swap: It'* a good time to put on a new accessory belt, since you'll have the old one off anyway. And while the belt is off, check the other pulleys for wobble.