Take the seal or better yet the old snap ring to a hardware store and match it up. If you use the seal for the size get it a little bit larger. Best to use the old snap ring or a piece of it to get the thickness.
Which key from the drawing is that snap ring? I was thinking he wants key 24, but he never responded. Key 24 is an internal snap ring. Your spec is for an external/shaft snap ring.
Also the top cover bolts are metric, so I would think the snap ring will be metric.